Reasonable price for a radiator replacement?

A lot, IMO, depends upon the quality of the radiator that will be used, the ability of the mechanic to do the job without screwing something else up,and the warranty for the work. To me, those variables are as important if not more important than just the price.

The last radiator I had replaced by a competent professional mechanic using OEM parts (but in a different type of car), I paid about a hundred dollars less than you were quoted.

You didn't say what engine or if they were doing a thermostat and hoses also. The job takes about 2 hours. That is a reasonable price for the job.