No, I'm sure you haven't paid over the odds - I assumed from what you said that you'd bought the Paragon product that includes the Chambers Thesaurus and a bunch of other stuff.
I was just surprised to see that Chambers on its own (no WordWeb, no Thesaurus, just the BRB) was available relatively cheaply from WordWeb software, as I had been under the impression that for Windows they only supplied it as an add-on to WordWeb Pro - with bells, whistles and matching price tag. I may have to get a copy to see what it's like!