Mercenary hiring costs were reduced game-wide in Update 2.2 last July. Other than guild perks and that one-time reduction, the only thing that changes the costs of mercenaries is their power: the higher a hero’s power climbs over time, the more it costs to hire them. And, of course, a hero’s power going higher means they’re getting stronger, so they can defeat more power in surge. The more power you beat in surge, the more gold you get from it. So, essentially, the more gold a merc costs, the more gold they can help earn for you.
That doesn’t help in your particular case, of course; I’ll also hire some mercs who would otherwise go unused to help out guildmates, which usually means burning them off in lost battles that don’t earn any gold. Other than such altruistic hires, though, I don’t think merc costs have gotten too high.
(And just to cut off any questions about it ahead of time, that first paragraph is referring to the costs/power/strength of a single hero. So, e.g., a Ralph that costs 250k gold is going to be stronger than a Ralph that costs 100k gold, but is probably not stronger than a Mulan that costs 100k gold.)
