You can find batteries anywhere from $10.00 all the way up to $35.00 for your Blade CX/2/3.  I find your better off with a lot of batteries vs just 1 or 2.  You will get more fly time per day if you have 4 to 6 batteries ready to go.  I have tested many batteries and have found them all very close to the same, and now most offer the new anti swelling lipos.  Look around and you should find someone selling them for around $10.00 to $13.00  Just make sure they are the new anti swelling lipo's.  I also highly recommend you read the battery warning and safety page on this site.  I could go on about safety all day long,  but it seems most of you really don't care.  I can only hope you care enough to follow some of the safety rules.  Remember it is not just you that could get hurt or even worse.  If you do nothing else, please never go to bed or leave your home with a battery left on the charger. This means even if it has finished charging make sure you have unplugged it before you go to bed or out, even if it's just for 5 minutes.