Difference between an RFID module and evaluation board?
I can't answer your "why cheaper" question, although market forces have a lot to do with it (up to and including the possibility that an eval board may be sold at a loss, to encourage design wins).

An evaluation board is intended for evaluation; it's a marketing tool for selling chips.  It won't necessarily meet all the requirements for a "real" product.  I would expect an eval board to work at room temperature but not necessarily over the full temperature range of the chip, to be physically clunky to build into a product (but easy to use on the bench), and to possibly have bugs.

A module is intended to be built into a product.  RF modules, specifically, are designed so that it's hard to mess up regulatory requirements when you use them -- so you can stick one into your product either without having to go through full compliance testing, or at least with the confidence that your compliance testing will pass.  They'll be specified for what environmental conditions they'll work under, and they'll be built with the understanding that if they don't meet those specifications the manufacturer will be in the black books of a lot of potential customers.  They'll be designed to be at least fairly easy to design into a fully integrated product.