Pretty simple to install Retropie yourself. Amazon has had the Canakits for $80 CDN recently, which has everything in this kit but the controllers and mouse. I would think on ebay you could get a DIY kit/case/controller for cheaper than one that has Retropi pre-loaded.
There's a Pi thread kicking around here somewhere as well.
This is the kit I put together:
Pi:
https://www.amazon.ca/CanaKit-Raspbe...ywords=canakit
Case:
https://www.amazon.ca/Raspberry-SNES...eywords=snespi
Controllers:
https://www.amazon.ca/8Bitdo-Retro-B...eywords=8bitdo
Note: N64 emulation still sucks. Most games aren't compatible, and you'll likely want a case with a fan.