The thing is, what players did was to build their entire army out of these foreign troops. This feels cheap and was generally unheard of in history.
The right solution for this problem would have been, IMHO, to make these units much more expensive so that you'd have to pick and choose where you use them.
They also should be mercenaries - to limit mass recruiting - instead of regular troops.
There you go, problem solved.
You either stay an unreformed horde and somehow get huge amounts of cash to support your troops - which is no easy feat when you need that money for high level advisors to combat your huge tech penalty, or you just reform and get better troops and need less money to actually do something.
And for multiplayer, I doubt that not reforming would ever be an option if those foreign units were mercenaries.
Increasing the cost of foreign units by such a factor would be almost as severe a nerf as outright removing the feature. To demonstrate this, even with only a mix of level 1 and 2 advisors, and an army of a size <75% of my forcelimits as Mongolia here, I'm still barely able to eke out 15 gold/month, and add colonies into the picture and it's barely above 0.
Even with lucrative income and trade from the Spice Islands, and owning + vassalizing the entire Steppe + Russian + Chinese + Persian regions, I can still barely support 3/4 of my forcelimit and make a meager profit. Adding the cost you suggest would basically make recruiting foreign units virtually pointless, since you'll almost never get those "huge amounts of cash".
Secondly, you don't need high level advisors to combat your tech penalty. The only tech you really need to worry about is military, and with a combination of excellent "No Child can Be Khan" rulers and a military NF, you can relatively easily stay on par with France, as I did here, even with only level 1 advisors for the first 250 years.