I wouldnt have thought it mattered that much so long as its a brand new one that meets the safety standards. For my first, I had basic foam mattress that was used for her cotbed and was used for abour 4 years before she got her full sized single bed.
This time round we have been looking at the firmer mattresses, again for use on a cotbed, and have been looking at ones costing sround the �50 mark from Babys R Us. Actually, as baby is two months old already, we had better get a move on with getting her cot lol