Even if these ports were $20, $10, or $5 it's still more than platinum is making off the games right now or in the future. The new copies of games that are out there were sold long ago.
For a game like Bayonetta, which has exactly one good version, a PC release would introduce the series to an entirely new group of gamers. Something important when a company is trying to push out a sequel. Publishers commonly do this with older titles, when new titles are on the way. Increase exposure of the series by any means possible (Sales, PS+).