Buying is incredibly cheaper than building, AND it usually results in a much better looking machine.
The few handbuilt machines I have seen that actually look as good as a factory cabinet usually had a cost sheet associated with them that ran close to $1000.
I just finished cleaning up a 1976 Bally/Midway Amazing Maze cabinet and it came out great. Cabinet was free, then I sold the monitor and boards for $100, I spent $20 on assorted paints and woods (had to repaint the back, interior, and top, and make a new base for it), and $75 on a 21" Kiosk monitor. Much cheaper than building, and it has a great look, full sideart, front art, and bezel art.
My other Mame cabinet was also equally free, and looks awesome. I simply could not build something so nice without putting out a major amount of cash.