A barebone or keyboard kit is going to be hard to find for $30 or less. The Tester68 is a 65% can usually be found around this price point, but other than that, there is the BM60 from KPR, but a black barebone kit is going to run $58.20 though it does have QMK.

Redragon has a barebone BBK530 Pro for $52. The Keychron V4 barebone is $53.

There is also the Eyooso Z11 but it has Outemu/Mill-Max sockets so only some switches will fit for $24.