Very good question and to my mind Audi, and in fact anyone that wants to compete should be obliged to offer for sale a car of the same type used by the works team at a capped price.
However I am sure Audi would tell you that the reason the cars are not sold to race teams (there are quite a few R8's in collections and at leat one race's in historics in the US i believe) is due to the complex nature of the car and the cost. I think the real reason is more to do with control.
There was a glimer of hope when Arena run the Gulf R8 and it looked then like a few others may be sold but it never happened. Goh where pretty independent I understand, but the others Velquox, Champion & Joest are all pretty much guided by HQ.
If I remember correctly when you brought your 956 it was sold much like any road car complete with a set of keys, however Porsche where pretty much alone in offering customer cars then in C1.
Only a couple of TWR Jags where ever run by anyone but TWR, most Nissans & Toyotas where run by either the works or semi works outfits. I don't think Sauber sold any merc's to anyone but Del Bello and those where normally well past they sell by date.
Lancia where the only team offer a package, and given the choice between a 956 or LC2 most went Germany.
Hopefully Porsche will be true to there word and some Spyders will be seen in privateer hands next season. Now that Porsche have a major stake in VAG hopefully some of that thought process will rub of and some R10's will be sold.