We're missing some important details: why are you taking it to the shop in the first place? Does the light come on? Is it making a noise in the belt/clutch area? Does it break so you can't drive it at all?
I don't think the shop is ripping you off; they wouldn't have replaced the belt the first two times.
Do you have a different shop you can take it to? $250 is a lot of money to replace a belt; the labor should be between 30 min. and 1 hour, and the belt around $100?
The BF guys know more about this than I do, but I think the BF650/750I have a problem with water splashing into the belt houing; if water repeatedly gets in and the belt begins to slip and you keep driving while it's slipping, the belt will be damaged.
Everytime I have taken itto the shop, the belt light has come on, I hear a clanking sound in the drive belt chamber, and the 4-wheeler goes into safety mode and won't go over 8 mph. It hasn't broken down completly to be un-drivable.
I also went back and looked at my invoices, the first time I took it in they said no problems really found, the drive belt switch was in the "ON" position but that the contact pin was bent. They replaced the pin and sent me on my way.
The second time I took it to the shop, the belt was replaced and that they adjusted the deflection sensor (or something like that).
This time around they are saying that the belt was stretched.
This is my first 4-wheeler that I have owned so I don't know much about working on it myself, thats why I have been taking into kawasaki, plus its still under warranty, but they insist on charging me this time around. I think once it gets out I am not taking it to this shop again.