The simple answer:
"There is no way that is both easy and inexpensive to add USB 3 to your Mac Mini that did not come with USB 3 built-in."
There are ways to do it using a Thunderbolt to PCIe adapter, and then get a USB 3 PCI3 card. But that would be a very expensive solution.
There are some hubs, some available now and others in the pipeline, that will do the job. But while not as expensive as a Thunderbolt to PCI3 adapter, they're not cheap.
I haven't seen a single relatively inexpensive Thunderbolt to USB 3 adapter on the market, but it's possible they exist and I missed them.
There are FireWire 800 to USB 3 adapters, and they're not terribly expensive. But you will be limited to FireWire 800 speeds (plenty good enough if you ask me, and much better than USB 2).
I'm still partial to FireWire 800 over USB 3, because USB still requires more attention from the CPU. For now, I'm buying storage devices that have both interfaces.