I think $50 doesn't quite get you what you need, but it may be close. Really, a good quality charger is well worth it, but you can get that down the road. The one that Pacman linked to will work, and it does say it can balance, I just questioned the quality of it. I actually have that one too, but after hearing horror (Boom! Fire!) stories of not using a fair quality charger with lipos, I just use my RC charger.
So, let's do it this way.....
Does it matter for the configuration of the pack? Nunchuck? Stick? Dimensions? I can do some checking and see what I can find for you. Another option for you is
www.hobbyking.com but you will have to add the connector of your choice. For some that is easy, for others it can be an explosion waiting to happen.
Also, another thing to factor in for making the change to lipos....You need a Lipo Sack for charging. Some will just use an ammo box, but really the lipo sack ensures that it contains the fire if the catastrophic failure happens. The lipo sack actually breathes or vents, whereas the ammo box can build pressue making it a bomb itself.
I guess I will summarize that to truly make the change to lipo, and to make it safely, you may need to up your budget some. Just my $.02.