I'd prefer it if mercs would be much cheaper than normal infantry, but would also have significant disadvantages, like lower discipline, military tactics and morale.
Uhh initially I'm fairly certain mercs were better because they were the only true career soldiers for a long time.
As they are in game, what they're supposed to do is act as a backup to your manpower pool. However, considering that mercs cost literally almost 3 times as much to upkeep compared to normal units (.21 to .58 per month for infantry iirc), I'm not sure how much more expensive you want them to be. I can guarantee you in the late game that an army of mercs isn't going to cut it against an actual major power, because the mercenary pool is very small- 30-40 mercs versus a forcelimit of 100-200 or more.
If it's any indication, when the AI fights wars with mercs, they'll take out as many as 10-20 loans just to upkeep them. So in other words yeah, mercs are pretty expensive. Even if you only merc infantry, it's difficult for even major powers to maintain a proper ratio of cannons, cav, and infantry without running a negative. And if you take money from someone after the war, then you get inflation, too.
Mercs can be frustrating to deal with when a nation abuses them but the economic drawbacks are already fairly severe. Even in the late game, if you have an army of mercs, that's money that could be spent buying +3 advisers instead.