Typically a monthly or upfront work fee plus a success fee at closing calculated on the transaction value. The structure is negotiated, and the engagement agreement is worth reading closely before signing.
More detail
Success fees in the lower middle market are commonly structured on a sliding scale that decreases as transaction value rises, sometimes with an incentive tier above a target price. Work fees, retainers, or minimum fees vary widely and are often credited against the success fee. The terms that matter beyond the percentage are the definition of transaction value, which should be clear about whether it includes real estate, assumed debt, earnout payments, and rolled equity, the tail period during which a fee is owed on a buyer introduced during the engagement, and the exclusivity and termination provisions. These are all negotiable. An advisor who will not explain their fee structure plainly is telling you something useful.