probally around 50, could be less. but if you buy pressure treated wood, it will cost much more (apx 100?) , but will last for much longer. if you dont buy pressure treated wood, make sure when/if you take apart your drop in you make sure to have the wood (espesially (sp) the plywood), leaning against something so it doesnt get soggy due to perciptation, or just cover it with a tarp.