Small, portable mixers with lots of stereo inputs are relatively rare. That plus the Teenage Engineering "tax" (think Apple tax but for synth-heads) is driving the price. A cheaper, but slightly less tactile alternative is the 1010 Music Bluebox:
I own an op-z and used to own some pocket operators. They will be the last teenage engineering products I own. Everything they release seems more ridiculous all the time. Just the ob-4 is enough for me to say "these are not products for synth people but tech nerds who want synths"
I also have to give TE props for their long term support and updates on the OP-1. I mean, just last fall they added USB audio in/out on a 10+ year old synth! Few other major synth manufacturer I know of hav as good a track record (Novation is actually pretty good in this respect as is Elektron, but for the most part they're fixing bugs not adding new features).
Compare this to another boutique synth outfit like DSI/Sequential -- my DSI Pro2 purchased in 2014 only got about 4 years of support before they dropped it like a hot potato. Tempest suffered a similar fate. Never buying another DSI/Sequential instrument again...
There are two USB interfaces on the bluebox -- one for power, the other USB midi devices.