How about a Hall effect sensor? You're right in the range where you can get one.

http://www.allegromicro.com/en/Prod...ifty-Amp-Integrated-Conductor-Sensor-ICs.aspx

Wow! Thanks very much. Just what I'm looking for.
The sell them on Pololu.

 

I agree with John P, take a look at these: http://dx.com/p/30a-acs712-current-sensor-module-158589

I'm sorry if I plug this particular store a lot, I just happen to surf through it often and I like their rating and review system, but they're cheaper on ebay