I just bought a once used Med BGE on Craigslist for $275 w/nest, tools and charcoal. It was in mint condition. The owner said he paid over $500 for it last year when he bought it.
My other Medium BGE was given to me by a friend, only used 3 times. Last year, he paid just over $500 w/nest and plate setter from the local BGE dealer in NJ.
At the Eggest in MD this weekend, the demo eggs for a Medium were posted at $489 which is less than buying at regular retail... \
Based on the above, regular retail is likely to be $500-$600 depending on accessories. So if you have the opportunity to get one at a good price... go for it !!! I'm very pleased with my recent acquisitions.