$5 or $10 may not sound like much, but since there are a few free license fonts already out there, isn't it better to stick with those?
Between Droid, Fontin, Gentium, Charis SIL, DejaVu, the Red Hat Liberation Fonts, OpenSymbol, FreeSerif/FreeSans, Linux Libertine, etc., is there really much left to be desired for ePub embedding?
There's a bunch more at sites like
Open Font library -- though that's got a terrible front end. Another smaller list
here -- I could have sworn I had found another better listing site... can't find it now...