I have one genuine EN-EL1 and 6 other manufacturer ones. They are cheap as chips on ebay. You do need the proper recharger though as the 'fake' ones dont force the battery to 'clip' into place and this can result in them not being charged.
Not sure what the total life span is and they dont last more than an hour or so (as opposed to 2 with my genuine one) BUT i can get 10 for the price of one genuine one...b