Coincidentally, I know both Corvallis and Petaluma (I went to OSU). They're like night and day, pricewise and every other way you could think of. I have a hard time imagining the two towns in the same thought. ;-)
Corvallis is certainly much cheaper, and it's approximately in the middle of nowhere. Pleasant town and everything, but it's central Oregon versus the San Francisco Bay Area. Petaluma is bound to be about 5x more expensive.
Regarding Sonoma County overall, it's true that the microclimates do change considerably from one town to another; even from one neighborhood to another. If that's important you'll want to check out individual houses, not just generalize by town.
Having said that, the farther west you go, the cooler, wetter, and foggier it can get. Towns like Occidental and Sebastopol will be a bit cooler than Sonoma or Petaluma. Not by much, but there's a difference. Locals describe the "banana belt" on one side of Sebastopol, for example, so even small changes in location can make a difference.
I would say that buying a house in Petaluma for $250 would be a stretch. I'm sure it's doable, but I can't say whether you'd like it. Many families move out here from the Midwest or Northeast and literally cry because they can't buy what they think they want or deserve.
Nearby towns like Cotati or Rohnert Park might be a bit cheaper, but again, who can say if you'd like it there? Come drive around (or play with Google Street View for a few days) and see what you think.
Because Petaluma is at the southern edge of Sonoma County, it's commuting distance to San Francisco, and that makes it more expensive than, say Santa Rosa 20 miles to the north. Santa Rosa's a bigger town (and the county seat) so it may be worth a look.