I have seen them change the plastic tanks on plastic ones. I have an older Dodge dually (95 model) which sprung a leak in the core. A dealer tech dropped a pair of Snap-On channel locks down in front of the radiator which wore through the core. Radiator shop cleaned it out, braised the core and pressure tested it for $65 (My cost) normally $100. New radiator at that time was a little under $400. Got a new pair of Snap-on channel locks out of it too. 
Also SOME radiator shops wont bother replacing tanks. The OP should call around to radiator shops to see whether they repair or replace. It would help to know exactly where the leak is.
transman