[Jack Simth]

Tack some expenses onto the renting of mercenaries (expenses billed to the "seller" of the mercs), and it might work and be less abusable; a per-unit cost of shipping them to the designated province (size dependant?), upkeep for the time away, a flat announcement fee, and perhaps a few other restirctions - e.g., the contract is for 3 months, but they are gone for 4-7 months (initial month to announce, stay available for 3 months if not hired, contract for three months = 4-7 months), and restrict the AI's so they don't hire player's mercs. Charge these expenses up front, regardless of whether or not the mercs get hired (they still had to announce themselves, arrange potential travel, get paid while they were away, et cetera). You can then set the price based on how much it costs you to hire them out. It wouldn't be very abuseable; few people are going to hire troops for much more than their upkeep costs would be if they could make them themselves (unless they are desperate); no-upkeep units, while not having the assossiated mantenence costs, still cost the full amount to transport - and they are usually either disposeable cannon fodder or have an expensive initial cost - cannon fodder nobody will pay much for, and something with a high initial cost is expensive to replace if your opponent sends them on a suicide mission the Last month of the contract. Moreover, running a mercenary company becomes risky buisness; if nobody hires your mercs, you lose money.

Of course, this is probably way too involved to be implemented for Dom II - III maybe.