I restored a Tempest and it cost me just under $1,000 all together:
http://home.comcast.net/~dark.paladin/tempestMy current project is another Tempest cab, but I definately can't afford to restore this one - so it's going to be a vertical MAME cab. The control panel on mine was already filled with extra holes, so it's going to have a T-Stick plus, 2 game buttons and 2 LED cone player start buttons. One thought about Tempest control panels is that unlike vector monitors, there are more panels out there than cabinets. I have an untouched spare tucked away in case I ever do restore the cab, and you can usually find them around for not too much money (especially unpopulated ones). The machine I restored had extra holes which I backfilled, and it's holding up fine.
I posted a question in the artwork forum asking for ideas of how to get the CPO made without the game instruction text, but didn't get any answers. I'm now planning on using a blue gridwork pattern.
My repro side art should be here this week - I've been kind of stalled while waiting for it, so I can't wait to get back to work.
Good luck on your project, and power to the Tempest MAMERS!