Ok, let's run with this:

I have a little chip sitting right beside me, 80MHz clock, 1/2MB flash, 32k RAM and something like 80 I/O pins, it costs about £5.

It's a surface mount component so add £15 for a board we can work with.

We need some way of getting data to it, say £5 for USB and/or £15 for Ethernet.

A keyboard of some sort, either discrete buttons, say 50pence each, or we could possibly use a USB key pad of some description.

Display options, the possibilities are vast. Off the shelf graphical LCDs are expensive and can be tricky to interface, say £50 for a 3 inch screen - have a look around for options. Or maybe just a set of old fashioned 7 segment units, say £1 each including glue logic.

Case, say £10 to £20.

Assorted connectors £20 <shrug> always more then you think.

Power supply for logic, a wall-wart is the safe option, £10.

The above is guess work but pretty much reflect UK prices.

These are your basic hardware building blocks. We now need a full hardware design and complimentary software design, as I still know feck all about CNC I'll leave these open for discussion :)