You can use a good old PIC, like the PIC24F(5-10$/chip) or some PIC18 have USB interface. You can use the free C compiler if you if you are afraid of asm.
I.E. PIC24FJ128GB202 or PIC18F4450 (less $)
Ready Board : mikroe(dot)com startusb-pic
Add a switch to use has a FG module or Update from pc.