Education costs on average US$1.25 a day per child in developing countries
It costs on average US$1.25 a day per child in developing countries (low and lowermiddle income) to provide a full cycle of preprimary through secondary education (13 years). The largest share of this cost, 88%, will be borne by developing countries themselves. The international funding gap is just 15 cents a day per child, on average.
Data come from two sources:
The Education for All Global Monitoring Report (GMR) estimates were used for (a) costs per child at each level of education in 2012 and 2030, as well as, (b) the financing gap (external financing over total financing required)
IMF figures for historical US inflation as well as an average of estimates for US inflation in 2017 from the IMF/UN/OECD and Economist
Our assumptions and calculations are then as follows:
As per GMR numbers, the costs of education are, with the exception of upper secondary, increasing over time (i.e. between 2012 and 2030). Since the GMR estimates are in constant 2012 prices, this means that inflation is accounted for, and the cost increases are real increases (i.e. more inputs are being used, or inputs, such as teacher salaries, are going up relative to the cost of other goods and services). If we assume costs increase by the same percentage each year, we can split this real increase equally across the 18 years, and figure out what the estimated cost of education is for each level in 2014, 2015, 2016, 2017... through 2030. This is shown in the table below.
Imputed annual expenditure values (constant 2012 US$)  

2012  2013  2014  2015  2016  2017  2018  2019  2020  2021  2022  2023  2024  2025  2026  2027  2028  2029  2030  
Preprimary  258  276  295  315  337  360  385  411  439  469  502  536  573  612  655  700  748  799  854 
Primary 1  195  203  211  220  229  239  248  259  269  280  292  304  316  329  343  357  372  387  403 
Primary 2  195  203  211  220  229  239  248  259  269  280  292  304  316  329  343  357  372  387  403 
Primary 3  195  203  211  220  229  239  248  259  269  280  292  304  316  329  343  357  372  387  403 
Primary 4  195  203  211  220  229  239  248  259  269  280  292  304  316  329  343  357  372  387  403 
Primary 5  195  203  211  220  229  239  248  259  269  280  292  304  316  329  343  357  372  387  403 
Primary 6  195  203  211  220  229  239  248  259  269  280  292  304  316  329  343  357  372  387  403 
Lower Secondary 1  301  311  321  331  342  353  365  377  389  402  415  428  442  457  471  487  503  519  536 
Lower Secondary 2  301  311  321  331  342  353  365  377  389  402  415  428  442  457  471  487  503  519  536 
Lower Secondary 3  301  311  321  331  342  353  365  377  389  402  415  428  442  457  471  487  503  519  536 
Upper Secondary 1  751  747  742  738  733  729  725  720  716  712  708  704  699  695  691  687  683  679  675 
Upper Secondary 2  751  747  742  738  733  729  725  720  716  712  708  704  699  695  691  687  683  679  675 
Upper Secondary 3  751  747  742  738  733  729  725  720  716  712  708  704  699  695  691  687  683  679  675 

Since a child obviously does not complete all her education in one year, and costs go up each year, we can't come up with a meaningful figure that talks about, for example, "the cost of an education in 2030." What we *can* calculate is the cost to educate a child who starts (preprimary) school in 2017: she would start preprimary in 2017, so we can add up the cost of preprimary in 2017 + primary 1 in 2018 + primary 2 in 2019 + .... all the way up to upper secondary 3 in 2029, using the values calculated in (1).
Based on developing country averages and the GMR goal of one year of preprimary, we assume the following number of years spent in each level:
Preprimary 1 Primary 6 Lower Secondary 3 Upper Secondary 3 Total Years of Education 13 *Notice that if one disaggregates by country, the total years of education and the years spent at each level will vary.The indicative path for a child starting school in 2017 is shown in yellow above. Summing the yellow shaded blocks, we see that the estimated total cost of a complete education from preprimary through upper secondary for an average child in the developing world is US$ 5,431.58.

As noted in (1) above, the GMR estimates are reported in constant 2012 prices, so we need to correct for inflation in order to report this number in today’s (2017) prices. We therefore multiply the number calculated in (2) above using measured US inflation for 2013, 2014, 2015 and 2016, and an average of estimates for 2017.
This gives us a figure of US$5,806.6 as the total cost in today’s dollars of a complete education from preprimary through upper secondary for a child who starts preprimary in 2017.
Finally, we figure out the financing gap, or how much of the cost of education is anticipated to need to come from external donors. Here we use the GMR total cost figures for 2030  estimated total costs of US$339.9 billion in education, of which US$39.5 billion or c. 12% are estimated to need to come from outside sources. This suggests that of the US$5,806.6, the financing gap is 12% or US$674.8.

In order to think about this as an annual or daily figure, we need to divide by (13 years) or (13 x 365 = 4,745 days) respectively. These figures are reported below (again, this applies to a child starting preprimary in 2017 and completing a full cycle of education through upper secondary):
Implied 2017 US$ Total Annual Daily Total cost of education (domestic + aid) 5,806.6 446.7 1.22 Financing gap (aid only) 674.8 51.91 0.14 
In order to find an average cost for all children who complete a full cycle of schooling between 2017 and 2019, repeat steps (2) through (5) for a child starting school in 2018, and 2019 and take an average of the three values, as shown below.
Implied 2017 US$ AVERAGE cost of education for child starting school in: 2017 2018 2019 Avg. 20172019 Average cost per child for full course of schooling, including preprimary (1+12 years) 5,806.6 5,940.6 6,080.9 5,942.7 Average cost per child per year 446.7 457.0 467.8 457.1 Average cost per child per day 1.22 1.25 1.28 1.25 Implied 2017 US$ AVERAGE financing gap for child starting school in: 2017 2018 2019 Avg. 20172019 Average financing gap for full course of education, including preprimary (1+12 years) 674.8 690.4 706.7 690.6 Average financing gap per child per year 51.91 53.10 54.36 53.12 Average financing gap per child per day 0.14 0.15 0.15 0.15