How much power does a mini PC add running 24/7, and how do I measure it?

I'm still deciding on a used mini PC (probably an OptiPlex or ThinkCentre) to leave on all the time as a small server. I keep seeing people say they're cheap to run, but I can't tell what that means in dollars on my own bill.

I haven't bought anything yet. I looked at the power brick label, but it only shows the max the adapter can give, so I think that's not the real use. Is a plug-in watt meter the cheap way to check, and is it worth buying one before I commit? Also, how do I turn the number it shows into a rough monthly cost? Sorry if this is basic.