Using the example posted, it has no monetary value as a postmark with the stamp removed.
It's "value" now is merely research data for postal historians to document the use of the various postmarks - early and late uses, etc. Someone collecting Denton, TX postal history might find it useful. I have accumulations of similar cancels which are saved and sorted for their date information.
Edit 1: A stamp collector might quickly call it "trash", a postal historian might find value in it.
Edit 2: I have 15 pages of Indiana cut-rounds mounted to notebook paper which are totally worthless as stamps, but I have mined numerous eku/lku items from them over the years.
My answer to the OP's last question. No, I would not invest very much time in them except for the few items deemed useful for data purposes.