Ktm Sx 125?

They are very fast and they are built very well.
I find that the aftermarket parts are much cheaper than OEM parts. In any event they are more expensive than jap bike parts. Top end is going to cost you double what a YZ would. Parts arent too hard to get though.
If you are used to lots of bottom, forget it, the 125 sx has little, but Boyyy does it have top!

Like Jaybird said, The 125sx is very fast but a little pipey. Parts are defenitely more expensive, but you can usually substitute aftermarket stuff for most high wear items (brakes, chain, sprockets, etc.) to save some money. The bike is very well built and things seem to last a lot longer than the Japanese stuff I used to run. The rings in my '01 lasted longer than the cylinder in my freinds '01 yz125!! (and I had more hours on my bike than he did) I haven't had any problems with parts availability but then again I haven't had to order anything real unusual. Another thing to note is how much easier it is to maintain and work on the bike. One small area of irritation is aftermarket / performance parts availability. If they will offer "it" at all, you have to wait until they get done makeing it for the Jap models. The suspension isn't as well sorted out of the box as some of the Japanese bikes but a good suspension tuner can fix that. Instead of throwing money at the motor (pipe, reed cage, etc.), just spend the money on the suspension. I don't regret buying the SX at all and plan to buy another one next year.