#2
What, exactly, do you mean by "guitar processor"?

Because there are all kinds of MFX pedals and portable digital emulators that have USB interfaces under $300.
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#3
The closest I could find is this:

http://digitech.com/en/products/rp360

But it's a bit pricey. Maybe you could find one used for about 100$? Honestly, this was a lot harder than I thought, there are a plenty of cheaper processors out there like the pocket pod and the Zoom Multistomp but their USB connection is only for updating/charging.

By the way, I remember paying about 200$ for my Pod HD, how come some older models like the X3 are a lot more than that used? Just something that caught my eye.
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#5
A used Pod XT can be had for under 100$, with luck maybe even a X3.
#6
Zoom G3.
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#8
zoom g3 does exactly what you want, you can probably find a cheaper one on ebay or whatnot, or just get saving.