You're buying two different things, so compare the work rather than the price.
Per-lead pricing usually covers raw or lightly worked information. Commonly quoted numbers are single digits to a few tens of dollars for an address plus owner name plus skip-traced phone number, and more when the bird dog has actually spoken to the owner and confirmed they'd consider selling. A confirmed motivated seller with a callback time is a different product from a row on a list, and it should cost more.
Pay-on-close means you pay a finder's fee only on deals you buy. Flat fees in the several-hundred to couple-thousand range are common, and some investors use a percentage of purchase price instead. You pay nothing for the misses, which is why sellers of leads price it higher per closing.
The part that decides which is better for you is your own conversion rate. If you buy one property out of every 40 leads, per-lead pricing at $25 each costs you $1,000 per acquisition. If you'd pay a $1,500 finder's fee instead, per-lead is cheaper. Flip the conversion rate and the answer flips.
One thing to sort out before either arrangement starts: write down what counts as a lead. If "lead" isn't defined, you'll be billed for vacant lots and out-of-area duplexes. And whether paying a fee contingent on a closing to an unlicensed person is permitted depends on your state's brokerage statute, so run the structure past a real estate attorney where the properties are.
