depends on what its worth to you.
http://www.blasterforum.com/engine-mods-swaps-129/yamasaki-yfsex-250-build-39172/
There a good start. I am by NO MEANS near fully complete and im several hundred dollars and many weeks into it and I do all my own fab work and welding... thats all just cost of materials and parts needed (although admitedly i could have saved a bunch a money and left some things off but whats the fun in that?).
Its certainly not a task for the weak hearted and will eventually take its toll on you and your sig other (if applicable) which in turn takes more of a toll on you. But if its what you like doing or are willing to invest into something of your "own" so to speak, its incredibly rewarding. My only regret is not going bigger on the motor, but im not sure the fram can handle much more with my riding style (if it is unbreakable, i can break it).
I say if your willing to get incredibly frustrated, pissed off, injured repeatedly, burned, cut, scratched and run bankrupt, DO EEET! it is a lot of fun and you have something totally unique to you and if nothing else, you can look at all the other suckers out there with their stock ones and smile a bit cuz of it.
but a word of caution (as if this all isnt a big caution), PLAN IT OUT! PLAN IT OUT! PLAN IT OUT! make sure everything is strong and clean and squared up. you can get seriously hurt or worse if the thing comes apart on you at speed (ask how i know this).
Not trying to discourage, just being realistic.
Good luck and i say go for it!
- J