Please tell us if you wish to monitor the power out of your radio to a Load or an Antenna or perhaps another amplifier?
Also any hints on the actual unit being tested, like frequency and output connection and if there is an antenna, feedline description.
Stuff like that.
This is just a very gross estimate of what I believe your costs would be depending on the way you want to measure your output.
In other words, how much do you want to spend?
For an in line wattmeter, count on maybe $20-50, Typical Ham stuff, not remarkably accurate, Or buy one from MFJ (link below)
For a good 300W plus Load maybe $300 used, if you are just measuring HF and not concerned about being exact, you can pick up a cantenna for a few bucks or buy one from MFJ maybe $50
https://mfjenterprises.com/collections/metersFor a good attenuator at that power maybe $500 or more used. I have not found any good cheap ones at this power level.
For an accurate "terminated" wattmeter" that will measure the signal out of attenuator maybe another $300 to thousands.
You can spend thousands of dollars,or just a few. Depends on what you want to do and how accurate you need to be.
In most HF radio applications you do not need to be very accurate
In development of a new amplifier, you probably need to be more accurate.