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: https://1010music.com/product/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"