Please check the sticky as it has a link to the Gearbox / Clutch FAQ

As for price.....there was a thread on here not too long ago about cost of changing the ISB. The main cost is labour for getting the box out.

if the ISB is on the way out...better to change it rather than have it fail and spew shrapnel around the box and make things worse. Better to be pro-active than reactive

Also check out your clutch and thrust (or throwout bearing) while you're at it and replace as necessary.

And yes.....only the reverse gear is straight cut, all the others are not (hence why Reverse is soo noisy!)