I did that, and it came out to $19 w/o shipping, $32.06 with (slow) shipping to the US, for qty 5. That breaks down to their usual $4 setup plus an extra $15 charge for the board size, but overall not bad.
To keep this keyboard-related: this is also roughly the size needed for the bottom PCB needed for a full-custom keyboard layout (a little wider, not as tall), so feasible for that as well. Dropping the thickness down to 0.4mm for that purpose doesn’t increase the cost at all.