a few numbers:
about 95% of all V8s sold were automatic models, so the manuals are quite rare.
Then, they were (and are) more prone to abuse by owners who prefer a "sporty" driving style, so finding one in good or very good condition is even more difficult than with the automatics. Manual transmission models tend to consume more oil than automatics, too.
And lots of those cars end(ed) up being parted out, with driveline and chassis parts serving the power-hungry Audi 100/200 tuner world, further reducing supply.
In general, manuals are more sought after and therefore more expensive than their AT counterparts. But that´s only relatively speaking - prepare to spend something between 3500 and 5000 Euros for a good condition one with average (around 250.000) km on it in Germany and Switzerland - the only markets where they were sold in noteworthy numbers. Prices above 5000 Euros, although being advertised, are almost never paid in real transactions according to my observations.
Check
www.autoscout24.com and
www.mobile.de for a market overview - or look at the
marketplace right here on this site.