We already have mercenaries - but the paying 1000 star coins for 50 soldiers is out of question. I think this feature - if implemented - would touch several issues we have in the game: too many settlers in star menu, too many gold coins, no use for tavern, disproportionate cost gap between R and M/S troops

a) Mercenaries will be hired in Tavern level 5 - this will require player to keep the tavern built all the time (also will drain some resources for upgrades - ie - use same amounts as normal barracks)
b) Mercenaries cost will be higher than training similar regular troops: ie. maybe x1.5 higher + certain amount of coins. This will drain even more resources. especially the gold coins (otherwise they keep devaluing everyday)
c) Mercenaries are not counting against total population, we can train as many we need (once hired, they will be moved to star menu); players will make use of the thousands and thousands of settlers we keep getting in adv loot and stockpiled in our star menu. Also, players who have low population cap, can make use of mercenaries to run certain adventures that require lots of troops.
d) Mercenaries HP/Damage will be slightly better than similar regular troops ie 10-15% higher. Make them interesting to produce for players who already have enough population / add more variety when attacking/blocking.