When you take him to a vet, he/she'll check for a microchip.
If no chip, then it's likely that you have been found by someone lost who was hunting for a home and loving owners. Might have been dumped by some cretin who didn't want him any more.
Never in our lives have we ever paid for an animal, we've always had rescue cats and dogs, and everyone of them were beloved by us for all their lives. And the love and protection was returned may times.