- generic firmware compatible batteries available on ebay for cheap
You can get cheap replacement batteries on eBay for less the half the cost of an OEM battery. But is it worth it?
In my experience, the answer is no. Typically, the battery cycle life (number of complete charge - discharge cycles a battery can perform before its nominal capacity falls below 80% of its initial rated capacity) is very short. Also the battery calendar life is also very short.
The advertised battery life is usually much less in reality - so you think the battery is going to go for 150 minutes, but find out it dies before hitting even 100 minutes.
I am all about making smart purchases so I can save my money. But I really don't think generic batteries are a smart purchase. Go with OEM - and only buy from a reputable dealer (because a lot of the "OEM" batteries on eBay are fakes).