There's a similar cart called MSSIAH for C64, bought it a couple of months ago for €49 (about $63), check it out here:
http://www.8bitventures.com/mssiah/ . It's still available as far as I can tell. The package contains the cart and a cool sequencer/synth software.
Excellent stuff.
If you want true video game-like sounds, buy a Yamaha FB-01 FM synth and a Philips PMC100, they're both cheap and really wonderful machines.
However, you don't really need special hardware for that nice lo-fi sound if you have a sampler or sampling program that is capable of downsampling to 8bit/2KHz. Try it with some standard square wave samples, FM basses or drums, et voila, there's your ung-tshak-ung-tshak-diggadiggaduggadump sound of the early 80s.
Sometimes I use my Ensoniq Mirage for that purpose. Best old sampling synthesizer ever made, can't go wrong with sampling + juicy Curtis analog VCF and VCA.