It just needs to be firm. No, you most certainly do not need to spend $300 on a crib mattress. Some people just prefer that the crib mattress be organic or hypoallergenic or whatnot. Here is the top rated crib mattress on amazon. Its $52.99 with free shipping. Just over 4.5 stars with over 2500 ratings and 1100 reviews. This is the second top crib mattress on amazon. Its $35.85 and prime eligible (woo!). 4.5 stars, 262 ratings, and 87 reviews.

Seriously, you might find out that your baby refuses to sleep in his/her crib for a while after birth anyways, so don't spend a much of money on a mattress if it isn't important to you.