How much should I expect to pay.

I have a 250 quasar engined bike with nearly 40k.

Haven't got much free cash for a few years but have a new job that only demands about 40 miles a week.

So I would like to try and get another couple of years out of her.

Is an engine rebuilt worth it?

What does it consist of.

what makes you think it needs one?

Its not always about the total mileage on a motor but the way its been ridden. Know of Quasar engines with higher mileage that have never been completely rebuilt

To have a 250cc single cylinder engine professionally stripped, inspected and rebuilt with any required new components would probably cost more than a replacement engine.

I don't know or think that it needs a rebuild. I was just thinking at 40k it might need a serious looking at. Not sure what signs to look out for that would suggest that kind of action.

If your compression is good and you're not burning oil, keep up with your regular maintenance and keep on truckin'.

Does it consume oil? Swarfs in the oil? Power loss?
Did anyone see material faults with a borescope (through the spark plug bore)?
If not, why do a rebuild!?

Buy a second hand engine off ebay when and if you need it

on my old B125 a tiny bit of metal came off in the cylinder and it lost compression that to fix alone cost £500 (have no idea why I bothered fixing it).

Water cooled engines these days last much longer, use good quality oil and change it as required. It will run many miles yet