Who even does this though? I kinda don't see Markiplier collabing with a random nobody even if you throw money at him. Besides if they do that, it kinda opens the door for everyone to start 'hiring' the guy/gal to collab with them and it might become annoying to them.
Well it's their money, I'd imagine it's not done often because collabs can be a positive thing to both channels as both audiences can see the other's content.
Markiplier wouldn't pay as he is one of the largest subbed channels and anyone would see that collaborating with him would be a HUGE gain in the channel. Really if if you have almost the same amount of subs as him, you'd still get a huge gain from his subs.
I'd assume you'd have to do it in private, perhaps on a messaging system. Think of it really like an ad sponsorship. You're paying for yourself to be in a video.
It's already been done before, like patreon users asking for donations for shoutouts.
Example would be berb and his patreon shoutout each month.
Heck even onision has one (although I do not like onision) where if you donate (250) you even get his personal phone number :I )
It's really no different than donating for shoutouts.
I mean on the plus side it might work, both sides are happy! One gets maybe 1000 and the other gets subs! (maybe)