Fixing hot pixels is a low level repair, they just use software to map out the bad pixels and then the cameras processor ignores those pixels. So, the cost is likely to be low.
This has the same basic effect as using Camera RAW to get rid of the spots.
The only real concern I would have about not having it fixed - is that hot pixels could make the camera harder to sell. On the other hand - the second hand price for a D300 is already pretty low so it may not be worth worrying about.