Personally, I don't think it's anyones right to charge money for emulation of a system they didn't invent - portable or not. I wouldn't just use the emulators without permission either. I just don't think it's ethical. I've always been opposed to this since the old days when all the emulators cost money (C64S cost about 70, iNES was 35, VSMC cost money but everyone pretty much just laughed at it). The research into how a machine works may be hard, but it's still stealing, and that money rightly belongs to Nintendo or whatever.
However, if the emulator had significant improvements over the original machine - eg features like a speedup button, save states, cheat functions...things that for the original machine you'd have to pay for the added functionality, if it is even available - THEN maybe I would consider it justified. For a bare bones emulator, regardless of compatibility, no way. And I ain't paying $10 or whatnot just for a GUI either.