Here's the mechanism, and then the part about specialists.
A buyer's agent has to be paid by someone. What changed with the NAR settlement is that buyers now sign a written agreement with their agent, before touring, that states the compensation, and offers of compensation to buyer agents are no longer advertised on the MLS the way they were. The seller can still agree to pay your agent, and often does, but it's negotiated in the offer rather than assumed. So a real deal usually looks like this: you agree to pay your agent 2.5%, you ask the seller to cover it in your offer, and if the seller says no you owe the difference at closing. Confirm the exact wording of any agreement with your agent in writing, and note that the standard forms differ by state.
On the number, @kestrel's math is right. Percentage compensation on a 200,000 purchase at 2.5% is 5,000. Flat fees for a defined scope, roughly 1,000 to 3,000, exist and some investor-focused agents offer them for repeat clients who source their own deals.
Specialists generally don't charge above market rate. What they do is different work for the same rate. An agent who runs a rent roll, checks the actual condition of the roof against your repair budget, and tells you the numbers don't work is saving you a purchase, not adding a line item.
Since you're coming from notes, one thing that catches people: an agent's commission is a closing cost on your side of the settlement statement, so it affects your cash to close and your basis. Talk to your accountant about how it's treated, because that varies with how you hold the property.