I personally have spent around £2500 for all the manual gearbox & related parts. Some I got second hand, some I got brand new from local Audi dealer.
Then there is labour to add on top of that. I havent done the conversion yet, when I'm ready I'm planning to go to Unit 20. Just have to sort out the pedal box. Thats another complication, there was no RHD manual pedal box.
My current idea is to get someone to weld on a A6 manual pedal box onto the A8 auto pedal box (with all the protruding bits machinde off).
Nollywood has found someone to make him a custom pedal box.
So its not a light undertaking, lots of parts gathering and waiting.
The main part is the gearbox, if you can get one of those cheap, you have a good chance of limiting parts costs to under £3k.
Look for gearboxed on German ebay, thats where I got mine for £500.
A guy in the states has done at least 2 conversions using Audi S4 gearboxes, so you dont necessarily need a A8 manual gearbox. The thing to watch out for is the final drive ratio in the gearbox to match to the rear diff final drive ratio.
Here is Tozo's post when he did his S8. For the A8 you either have to re-code the ECU so it doesnt look for the Auto gearbox signal, or trick it into getting a signal, thats what Tozo did on his A8.
http://forums.quattroworld.com/a8/msgs/12898.phtml
All cars have their problems, before I found this forum and bought a S8 I researched E39 M5 (vanos problems, needs frequent valve shimming), and Mercedes SEC560 (not too bad but a bit wallowy round corners, rusty). Later I also looked at Mercedes E55 (had some timing chain and head gasket problems, rusts from new, water based paint, check Wikipedia).