There is a great line about specs, quality and cost that you can fix 2, but the third is dependent on the values of the other two.
You can't get anywhere close to $250 for a machine that can accurately cut wood, let alone aluminum or steel.
If you were a really talented machinist who can make parts really accurately from blank steel, you can trade a whole lot of your time for cost. Other than that, you either make the frame from 8020 extrusions, in which case you can get a strong frame with decent accuracy, or you make it mostly out of MDF, in which case it is cheap, but not very strong and accurate.
Even with MDF, most people report spending north of $1000 on a complete CNC.
Keep reading, and keep saving your money. There are lots of build logs of folks who have built something with your specs.