Instead of charging for a new something just reactivate the RFID tag. Easy peasy.
I'm going to assume that MAYBE part of their reasoning for not doing so currently could be related to a security or technical concern.... such as either concern over the life expectancy of the RFID chips [either design life due to a powered active design, or maybe even concerns of robustness from things like people washing their mugs at home in a dishwasher and potential damage to the chip depending upon it's placement in the cup].... or the risk of someone hacking or cloning the RFID chips within cups that have spent significant time outside of Disney's possession.
There could also be a training, customer service, or system limitation in place currently that could make it difficult to simply reactivate an old mug.
Training: adding another set of complications to the cashier training regarding verifying the mug is RFID compliant and then how to ring up and reactivate it for whatever time frame the customer desires.
Customer Service: Currently if your mug doesn't read or work, Disney can just swap it out and give you a new one since the old one was defective on their watch. If a re-activated mug doesn't read or won't work, It brings up a whole slew of technical support issues [why won't it read/work], and would limit their ability to 'make it right' easily. [refund your reactivation price? ok.... but then you would need to pay more to get a new cup.].
System Limitation: How is the POS and RFID System set up? Will each cup have a totally unique RFID identifier? Or because they are cups with a max 14day 'lifespan', is Disney just going to cycle between a small number of RFID's with the basic assumption that the odds of having 2 guests on site, with a new active mug, during the same 14 day period, would be astronomical.... but the odds greatly increase for a ID Collision if older mugs are brought back onsite after their initial expiration.
Or maybe it's a POS limitation in how valid ID's are stored in the database and activated. Maybe a Central warehouse or each resort loads each shipment into inventory for the POS system upon receipt, with old inventory set to either self destruct from the database after it's authorization period ends or be 'cleaned up' from the database after a set time period. Either way, it would make sense to try and keep the database 'clean' in order to improve system performance and safe on storage needs. It would then also be logical that maybe the POS systems the cashiers use are unable to add an RFID to the database, but are only able to update an existing record.
Now.... I could also see most of these issues becoming much more of a moot point as they expand the Magicband system. They may then decide to start tying your drink refills to your Magicband (RFID Tickets) instead of the mug itself, which could very easily make it much more likely that they could offer a "old mug use" type pricing.... even if it turns out to be a direct fountain-initiated per-refill charge. [IE. You have your room charge privileges or charge card tied to your Magicband, so for a drink refill you could bypass a cashier and just walk up to the self serve soda fountain. The system would then read your Magicband and charge $1 to your account for a one-time-refill ]