iPhone 3G battery issues

I've had my iphone 3g for about 17 months now, about a week ago the battery started dying extremely fast. I'd charge it fully and it would die within an hour or two without me even using it. I took it into an Apple store this weekend and had someone check it out and the guy said my was basically at the end of its life.

My options at the store were to either pay $150 for a new iphone 3g or pay $80 for a new battery, both I found not worth it considering I only paid $50 for the phone.

Is there anyway to just buy a battery for the 3g and replace it myself for cheaper?