you can take the gamble on those "it might work and it might not work" boards on ebay for $100 or less, and you can probably use that board as a paperweight or something to scratch your nuts with.
that's what happened to me. then I had delusions of grandeur and bought a junk board with the CPU ripped off thinking I could do surface mount soldering. I wound up reselling them to someone that needed RAMs.
personally, the best decision I made was buying my first KI board from quarterarcade, I dropped about $230 on it, but at least then I knew it worked. I upgraded the game rom on it and eventually bought a RevisionX flash kit for it.
it'll take some effort to hunt down, but occasionally on ebay you'll find boards that are listed non-working that have broken hard drives, as evident by the idiots selling them posting blue and red screens. I've seen them go for what you consider your budget. I wouldn't pay any higher than $250, there's some other sellers on ebay that are outright price gouging (try $400), but I guess when they have nothing else better to do in life they gotta make money too, ya know.