Atomiswave is rad, but you need the right setup for it. It needs to be played in 31Khz and with a dedicated five button or more CP. I run one in a NetCity and the system looks amazing. When you see games like Fist of the North Star or Rumble Fish 2 (even KoF XI) running in 31khz, they're a sight to be seen when you consider the hardware they run on. It's hard to believe it's just a cart-based Dreamcast sometimes.
Legit games are decently price for all but a few titles, namely SS6, MS6, Rumble Fish 2 and Guilty Gear X v1.5. Everything else is in the sub $100 range a cart, and even then you can find many games on eBay in the $30 range if you're patient. You do have to be wary of bootlegs, though, as even most sellers either can't spot them or are crooks and won't tell you what you're getting. You'll have to educate yourself on how to spot a fake before you drop some coin. Especially on Sam Shodown 6 or Metal Slug 6, as spending $200+ for one of those and getting a fake won't be fun.
SS6, MS6, GGX1.5, RF2 are in the $200 to $200+ range, everything else is decently priced, including Fist of the Northstar which you can find pretty regularly under $100 for a legit cart.