Your payment gateway integration cost will vary based on your needs and will be calculated in hours of developer work. If your ecommerce architecture is coherent, adding a hosted payment gateway will be a matter of hours and might cost you $25-$50. Meanwhile, implementing and customizing an integrated gateway will take more time and effort on the side of the development team, which will be reflected in costs. Some providers also charge payment gateway API cost (e.g. $17.50 for Amazon Pay).
Reach out to us at Elogic to get a more specific estimate for your integration project.