There are two o-rings that mount in inset rings inside the pipe flange. Over time, they wear out. It's about $10 or so for an FMF two o-rings / two pipe springs kit. Replacing the OEM copper gasket that fits between the pipe and cylinder is a good idea at around $8. Usually, it is about 15 minutes work for someone used to the KDX, 30 minutes or so for someone not familiar.
It's hard to say if $2,500 is a good price for a bike we haven't seen. That seems a touch high, but not super high, if it is in great shape with low miles. Check the air filter for cleanliness.