Unfortunately the start up cost for electric isn't cheap...but you can do it fairly reasonably and I think the long term price ends up being less than nitro. All helicopters these days run LiPos, so you're going to need a new balancing lipo charger. I have the iCharger 306b and it's great, if you order from EPbuddy they'll give you a free parallel charging board with it that can significantly reduce charging time provided you have a capable enough power supply.

If you're looking to go 50 size, battery cost is going to depend on the type of flying you want to do. A helicopter like the Mikado Logo 500 suggests a 5000mah 6 cell lipo which you can get for about 60 dollars or so a piece. Some people run 8 cell setups which will cost a bit more but give you a little more power for aggressive flying. Going up to a 600 size or larger most people run 12 cell setups which are actually two 6 cell packs connected in series.

For my trex500 I have three 2650mah 6 cell packs currently and it's enough for me. You'd need a bit more to do constant flying but if you have some other helis you're probably not too worried about that.