The pro-am versus amateur couple debate will go on forever. There are advantages and disadvantages to both. It's good for you to know that finding an amateur partner and competing with him is an option and is cheaper. But the choice is yours (and often depends on circumstances; I get a bit riled up when men blithely say "find a parner"! There are many more women than men looking for partners, at least in the U.S., so that's easy for a man to say!)
That said, typical costs paid to an instructor for competing pro-am include:
--a per-dance "floor fee" for each heat you enter. This is sometimes buried in the entry fee the studio quotes you, for example, the comp charges the studio $25 per entry and the studio charges you $40. The more dances you enter, the more expensive the comp will be.
--the instructor's registration and daily entry fees.
--the instructor's travel expenses (transportation, hotel, sometimes meals)--if the instructor brings more than one student to a comp, this cost is usually split among the students.
--a "pro fee" or "studio fee"--a set amount to compensate for the income lost while the instructor is away from the studio and can't teach other students and for the fact that the instructor is sacrificing time off, personal obligations, etc. to go to the comp.
Each of these costs can vary in many ways, so I'd be surprised if any two students at a comp were paying the exact same amount!
Competitions are a major money-maker within the ballroom industry. The comp organizers are looking to make a profit, and so are the instructors and studios who take students to the comp.
If you can afford it, competing can be a lot of fun, but you do need to go into it with your eyes open.