AffiliateWP

AffiliateWP is the default answer for running an affiliate program on WordPress. More than 30,000 businesses use it, and because it is a plugin rather than a hosted service, the affiliate data, the commissions and the tracking all live in your own database with no platform fee on referred revenue.

Licences are annual and priced by sites. Personal is $149.50 for the first year on one site with unlimited affiliates and commissions and the core tracking, payout and integration set. Plus is $199.50 for two sites and adds multi currency support and growth tools like leaderboards, signup commissions and bonuses. Pro is $299.50 for five sites and brings advanced fraud prevention, lifetime and recurring commissions, multi tier commissions, the affiliate portal and landing pages. A Growth Bundle at $349.50 adds RewardsWP for loyalty and refer a friend across ten licences. Those are introductory prices; renewals are at full rate, so budget $299 to $699 in year two.

There is a 14 day no questions refund.

The trade offs are the usual self hosted ones. You are responsible for hosting, performance and updates, and tracking is only as reliable as your caching setup. In exchange you pay no percentage of revenue, ever, which over a few years is the cheapest way to run a large program.