Yes, there will be a gain.
The shop I'm having port/coat manifolds for my business, McCoy Motorsports, has been building high power Corvette, F-body and Mustang engines for practically 20 yrs and they claim they are seeing 15-20 HP just from coating (internal and external).
The internal coating promotes more flow, keeps the metal of the manifolds from getting too hot, and promotes a cooler engine bay, which in turn allows your engine to run cooler (how much depends on what'* been coated).
All-in-all I would have to say, yes, the internal and external coating is worth $75.
BTW, are the manifolds ported/will the be ported before being coated?
If not, I can have them ported for $80, but the problem is I don't have any 00+ mani'* ATM for cores, so you would have to provide yours.