Mercenaries...

[tinkthank]

Yes, the AI bids on mercs is suboptimal at the moment. This is a known issue, but it is not known whether it is considered important enough for the devs to work on a new re-hire pricing scheme.

As to tossing away half- or mostly-used mercs: I think many people do this. But alternatively, you can just let them get hired by others and have them waste their money (see above).