A-men brother! 2 strokes are 130 for a new piston kit, only diffrence is a 4 stoke is 50$ more, but it lasts 3 years practically if you can shift it well. A 2 stroke runs best if you switch it every 1 year, so you do the math
250F-$180
250-$390
To see 2 smoke buried at the track-PRICELESS
hahaha sorry, had to add that last one. Personally I like all bikes, 4 strokes have been proven cheaper in my mind. Sure, you gotta pay for valves and springs but with proper care you can make them last too. So around $550 I think to rebuild the whole top end and valves if you do it yourself, but then again, by this time, your still under.
my 200 will do the same amount of hours as a 250mxer....... piston was $110, less than 150 total...... The 250 will need valves about that time too, and cam chain soon after Entire top ends with valves is over 550 typically too... But ride what you want and carry on....