Custom keyboards are expensive. The benefit of the GMMK Pro is you can easily install new switches as it's hotswappable. This lets you have a lot more options for switches than most companies give you. I personally find MX Browns kinda meh.
I'm using a Hexgears K705A - it cost me £60, with BOX White switches and cheap double-shot ABS keycaps. That's less than a bare non-pro GMMK. I replaced the keycaps with a pretty expensive set of sculpted PBT's and it still worked out less than a bare GMMK Pro.
And my space bar doesn't stick :P