A basic queen - size memory foam mattress can be had for around $ 300 - 350 US, but to ensure you're getting a decent quality mattress you are best to start looking around the $ 700 mark.
After a bit sharing in a queen bed with me, near DD turning 2 we side - cared the crib mattress to the queen (both on the floor with no box springs), and attempted to get DD to start the night there (didn't happen).