Theoretically you get a boost from a unique IP because it's deemed a mark of quality according to one of Google's patents. How much of a boost we don't know. If you are interlinking your domains then the more IPs the better; however it's less of an issue if you don't put sites in the same niche (linked together) on the same IP. Your IP usage really only depends on your site building and interlinking strategy. Long term site, interlinking between sites => unique IPs, preferably on different hosts. Shitty short term sites, not interlinking, mostly external backlinks => don't worry about IPs too much.
I used to be militant about unique IPs but personally I see little difference in unique vs shared; however I don't generally interlink my sites heavily and mostly get external backlinks rather than recycling my link juice. If you have hundreds of domains then it gets expensive fast. Generally I just set up a new virtual account, build out as many domains as it will take, and use whatever IP system the host provides eg. if they offer cheap IPs then I'll use unique IPs (or sometimes a few unrelated domains on the same IP), if they only give you one IP or mix you in with lots of other customers on the same domain then that's fine too. Mix it up.
Really though you're way overpaying for IPs at $3 each. You should be paying $1 per IP max if you shop around. If your host is charging $3 an IP then they're probably overcharging you for other stuff too.
