lone star balsa in lancaster texas does a pretty good job of "selecting" balsa I believe. you can compare the weights of the materials against each other. the link kevin posted has the cu. ft. weights of the rohacell and the balsa house ought to get you close with theirs. I believe the absobtion of epoxy into the materials would be similar due to the "cellular" construction of the rohacell. I know in the few field repairs I have participated in that was the case.

http://www.lonestar-models.com/