$50 a month... you think seriously, man! Well, get your own hosting, domain name, and put a good script in there - this way you'll be sure you have control over it, and you'll pay one-time fee for the cart that way, and further you'll pay only for hosting & domain, but that's WAY cheaper than $50/mo. BTW, if you buy Avactis shopping cart, you get 1 year of hosting free, so you just need a domain and you're ready to go The devs @ Avactis will help you installing if you're not too geeky — was easy for me without help though....
I would advise Viart. It's a good all around e-commerce shopping cart. I've used them for my sites and been very happy with them.