I found some values determined by some number theory people. I don't know if their numbers are accurate or not but it looks like they went to considerable effort to get valid values.
This short document gives the values they came up with. Note that they created their own scale that doesn't match with the standard Q=9, R=5, B=3,... scale of standard chess.
http://www.symmetryperfect.com/shots/texts/values-capa.pdf
They explain their methodology in this long-winded document that I didn't have time to read.
http://www.symmetryperfect.com/shots/texts/calc.pdf
Just out of curiosity, does anyone know the relative values of the pieces in Capablanca Chess?