Rebuilding a transmission...

[sbddude]

What problems can rebuilding a transmission fix? How much does it usually cost (parts and labor)?

I have a honda prelude with a 5 speed and when shifting into 3rd at above ~3750 RPM the gears will grind. My guess is that the synchros are worn out. Am i I correct? Will rebuilding fix this?

[MagicRat]

It sounds as if you are correct.
There is not much to rebuilding a manual trans other than new syncro rings, bearings, seals and fluid. Other parts, like shifter forks, gear sets etc are only replaced if worn or damaged.

However, before rebuilding it, drain all the oil and take a close look at it. Look for metal sparkly bits, and consider sending a sample off for some professional oil analysis. Excessive traces of steel or brass (from syncros) may help identify problem areas. Fill it up with fresh oil and see if that helps.
Also, try shifting a bit slower, that is, when shifting, use less pressure on the lever when engaging another gear. Your shifts may take an extra second or so, buts its much easier on the syncros, and you might just get many many more miles on the trans you have.
Also, try a bit of double clutching and matching revs, especially on the downshifts. Sur eits more work, but it might make you a more proficient driver.