Lots of useful threads on this exact topic lately. Read further down the page, or do a search. My recent experience exemplifies two main points:
1. Playing time will depend much more on using your system smarter, vs. filling your boat with batteries.
2. Not a good place to try and save money.
I don't have enough personal knowledge to comment further, but I did listen to some very smart people from these boards and those discussions can be found easily.