True--if you're looking for a newer, factory-warranteed daily driver, the V8 S4s are mid-high $30s and very hard to find.
The real bargains are in the older twin-turbo models ('01-'02) which are frequently in the low-mid $20s. They generally fall into two categories:
1) Post-blown turbos. Have either had the turbos replaced with stock under warranty, which is fine and you have new turbos. Or, upgraded with larger turbos for the same price, which is fine and you have 350HP and more durability.
2) Pre-blown turbos. Most people who have maintained their cars in stock condition have purchased an aftermarket extended warranty specifically to pay for replacing the turbos when they die. I would never buy an S4, on original turbos, without a warranty. Fortunately, the aftermarket warranties are generally transferrable for a fee.
We've had an '01 S4 Avant for 70k miles. It was in a bad accident two years ago and we chose to keep it even after $20k worth of repairs. We looked hard, but there is nothing available in its class that comes close.