I could set you up with an entry level Rb67 kit, prism finder, 120 back & 90mm lens for $300.
I could throw in some film (think maybe an old propack of Ektar 100 and a few rolls of B&W) as well.
I know its more than you wanted to spend but its in the same league and it would get you into a system and a non-FSU MF SLR.
If you decide to go with a 6x6 SLR and you get a former Soviet Union body, make certain that your trusted seller has actually used it with some film, a lot of FSU glass is amazing (I love the 80mm f/2.8 Volga) when they got it put togather right, but the bodies are of mediocre quality if they even work.
If you like 6x6 more than you need an SLR, consider a TLR. Rolleicord, Autocord, Mamiya, or Yashica 124 are all great choices and often obtainable for under $200.