How important is mattress quality?

chelseamb11

We found a crib we love (at Walmart of all places) and they are doing an online deal where you buy the crib (it's on sale even) and you get a mattress for free.  The mattress itself isn't the BEST of qualities (it's also not horrible either), but there's always the option for adding a mattress pad instead of spending another 70 on a mattress.  Or is the 70 worth the investment in that a better mattress is better for the baby?