You'll need to consider license fees, milestone payments and royalty fees. License fees are paid up front. Companies often are reluctant to pay hefty licensing fees if they don't know how successful the technology is going to be. Milestone payments are paid as the product develops. For example, if the product is a drug, a payment is made as the drug passes through each clinical trial.
Royalty fees are a percentage paid based on net sales.
You'll likely need the patent owner to support and maintain its technology, so be sure to clarify the specifics. Because technology can improve quickly, you'll also need to secure an agreement to receive updates and enhancements. Protect yourself from damages in case others in addition to the seller turn out to have claims to the technology. If you get sued, and the company can't back it, your indemnification means nothing. To avoid this problem altogether, investigate the ownership of the patent and technology thoroughly during the due-diligence process.