it might be easier to build an all-new starbase
Agreed. We never got an indication that Starfleet or the UFP would find it difficult to erect a starbase in the technological sense. Politics drove them to adopt and adapt Terok Nor, and politics may have played a role in obtaining Farpoint, too. But for all we know, constructing a starbase from scrap is fairly trivial - perhaps even simpler, cheaper and faster than building a much smaller starship because the base doesn't need to move at warp.
What I find quite interesting is the Cardassian side of this equation. Why did they build two seemingly identical stations? Terok Nor was supposedly optimized for controlling and strip-mining Bajor. Was Empok Nor supposed to augment Terok Nor at Bajor, but was abandoned in mid-tow for some reason? Or was it destined for another conquered planet very similar to Bajor?
OTOH, if its location in the middle of nowhere was its intended assignment, why was it of the exact same shape and design as a station intended to operate on fairly low planetary orbit?
Regardless of whether towed in place or built in situ, Empok Nor suggests that even the Cardassian Union can abandon large structures of that sort without much concern - this further implying that their construction is a relative triviality, even for entities weaker than the UFP.