I had the rear gearbox mount replaced on my 8v GTi about 2 months ago. I had a good local garage do it. The mount cost me £23ish inc. VAT from GSF and garage charged me £35 for labour (about half to 1 hours work for them I reckon

). I guess it depends how mechanically minded you are, having the correct tools (good trolley jack, axle stands etc.), how much time you've got to spare etc

.
In my case I had a look a through my Haynes manual to see what was involved, this was a bit flaky IMO. So rather than jack it up, get underneath it, put the stands in the wrong place to take the engine/gearbox weight and risk having a gearbox drop on me, I decided to leave it to a garage with all the right gear to hand

.
As it turned out, the chap who did mine had done this very mount on several mk3's before

. Apparently it's a common wear and tear job on mk3's.ÂÂÂ
The mount on mine was so worn out, that the excessive movement of the engine when changing gear or under hard acceleration was causing the whole exhaust system exhaust to move backwards and forwards, banging against the underside of the car

. Very irritating.
If in doubt mate, leave it to the professionals

.
Phil