a set price
An established, inflexible price (for something). They offer a three-course meal at a set price of $60 per person. I'm sorry, but that's a set price. I'm not willing to negotiate.
set one's (own) price
to name the (relatively high) price at which one is willing to sell something. If you have a first edition of Milton, you can almost set your own price. They are in great demand by collectors.