Yep. All you need is the cylinder and head. The base gaskets are the same, but you'll need a new one. One of the head o-rings is the same as the 300, ther other shared with the 200, can't remember which is which though. The exhaust spigot, powervalve assembly etc, all come off the 300 cylinder and will slide right into/onto the 250.
Price wise, I paid approx $800 from go fasters for the S3 Cylinder and head with 3 x inserts.. I'm sure it could be done for less if shopping around and considering second hand parts and maybe the cost of a replate.
I also see you have a 2012, and from what I know the 2013 has the new refined cylinder which is much stronger. Haven't confirmed it for the 250 but definitely for the 300. motocrosscenter.com had pretty competitive prices on OEM cylinders and heads too. Gofasters might be able to match prices. I'd go for the newer porting personally.