I've had a mattress last 90 days--paid $6 for it. Air mattress. Great mattress, until it leaked.
Next mattress was really cheap, because I needed it to last for about a year only before I'd be throwing it away. It lasted a year, but the last month or two it was a seriously uncomfortable mattress. I think I paid $250 and sold it for $50.
Our current mattress we paid about $500 for (full) at Costco. It's alright. Will be relegated to a guest room very soon.
My next mattress will probably be an Ikea futon mattress if it comes big enough and I can convince my wife that it is "ok" to do that. She had it for about a year and we still sleep on it when we visit friends. It is extremely comfortable.