Your iPhone 7 will support iOS 15 when it rolls out this fall. That does not mean your phone will fully support all of the features in iOS 15 as some are based on specific hardware technologies that are not in your iPhone 7.
As to the best price, I have no idea and that can vary from day to day. A better yardstick would be the best VALUE and although there are plenty of pundits who will wax eloquent on that, in my opinion it is something that can only be determined by the purchaser based on their unique needs and desires. For example I place a high value on the latest "gee whiz" technology an others may place a higher value on low cost. Neither is wrong, just different and unique to the individual.
