There is a theory that the Corvette is a supercar by right -- that its high performance warrants its inclusion in that exclusive category, but that its lack of refinement and rarity is what holds it back in the eyes of the public. If that theory is true, then the Cadillac XLR-V should count. Under the skin, it's all Corvette, and even today, 450 hp is nothing to laugh at. The rest of the car is all Cadillac, and is sitting on a plate for anyone ready to eat a mid-$40k slice of cake.
*NOTE: These are all available for under $50,000, but the cost of maintenance varies greatly, so making sure you buy the most well-sorted example within your budget is paramount.