In this month's TES column (I'm calling it a column, it sounds better than a blog), I call parent-teacher meetings in Bangladesh "maybe one of the most cost-effective interventions ever studied". Here's the maths behind that claim.
First, the intervention found 0.377 standard deviation effect on Grade 5 scores and 0.141 standard deviation (not statistically significant) effect on Grade 3 scores. If we take the average of those, that is 0.259. That's equivalent to around 1.7 extra years of school (based on Evans & Yuan's estimate that 1 standard deviation ~ 6.5 years of school).
The cost was $3 per student over the two years. The author Asad Islam does the conversion using only the 0.377 effect size for Grade 5, writing "Thus, the cost per average 0.1 SD increase in test scores per student is $0.66 or $1.58 for the full program over 2 years."
J-Pal put together a list of the cost-effectiveness of different interventions on their website, now gone, but replicated by Romero, Sandholtz, & Sandefur in the Liberia Partnerships Schools paper (copied below). Islam's $1.58 per 0.1 SD increase is equivalent to 6.3 standard deviations per $100. If we use the more conservative estimate of 0.259 SD (averaging across Grade 5 and Grade 3 results) that still works out at 4.3 SD per $100 spent. That lower estimate still puts this intervention at third place in the ranking, so there you go: "maybe one of the most cost-effective interventions ever studied".