I know it's not what GP is asking for, but you can also integrate RetroArch with Steam - it's on the Steam store. My kids can play steam games and emulated games now without having to run or figure out two systems. Also a high powered PC plays certain games much better.
The drawback to RetroArch is that the UI is non-intuitive and it's a bitch to configure.
a quick search for retropi on amazon brings up listings for close to $300, for a pi in a case with a $15 controller. looks like there is business to be made by installing a distro on a $35 pi and marking it up by ~$250.
The drawback to RetroArch is that the UI is non-intuitive and it's a bitch to configure.