Apple hardware in general keeps its value well compared to PC's, but if the Mini is discontinued, or massively upgraded, then resale value could drop.
I Would be worried about the power of the Mac Mini, and would atleast look into something like the Macbook, or iMac, as a good alternative.
I Would be worried about the power of the Mac Mini, and would atleast look into something like the Macbook, or iMac, as a good alternative.