M.D.
My mom just bought the Colgate Classica I mattress for her crib .. I have the same one and it has been great so far (my son is 6 months old). It retails for $100 + $10 UPS Ground shipping at www.babycatalog.com. It is the recommended mattress from Baby Bargains (a leading consumer review book), and it only weighs 7 pounds vs. the much heavier spring mattresses, yet it is very firm and safe for SIDS, etc.
http://www.windsorpeak.com/babybargains/default.html
http://www.babycatalog.com/cgi-bin/WebStore/indexNZ.cgi/m...