What does the "c" in bicycle tire size mean?
It doesn't mean anything, really. It's a historical relic.

At one point in time, "700c" was a standard tire/rim size, where "c" indicated the bead seat diameter and "700" was the diameter, in millimeters, when the tire was installed and inflated.

That "700c" tire size has a 622 millimeter bead seat diameter. There's an ISO standard that's often listed in small print where those tires are all '25-622'. This is the same as "29er", though usually tires marketed that way are substantially wider. "700", "622" and "29" are really all the same size.

All those tire sizes you list are the same.  If there were different prices for the "700x25" and the "700x25c", it's because there was some other difference between the tires, like model, threads per inch, bead type (wire or kevlar), etc...