It's worth it. I'd say it'd still be worth it at double the price.
The cab is the most valuable part. It's easily worth $100 by itself. The monitor and board can be replaced if necessary.
You will have all the help you could ever need from the guys here on BYOAC. Old monitors were very easy to work on and if you have a little skill with a soldering iron I'm sure you could get it back up and running in no time.
If all else fails Asteroids works 100% in MAME. You could put a 19" raster monitor in there and a PC to get the game up and playing. It's not the best restore option, but it would be the easiest route. If you chose to do this then I would recommend decasing a a 19" PC CRT, as these can do very high resolutions that will make vector games look more authentic. I figure you could go this route and have a working cab for less than $200 including the price of the cab.
There are a few physical differences between a raster tube and vector tube, but I think they can be swapped. In the case of Asteroids it used a black and white monitor so I figure you would need a tube from a black and white raster monitor. When you get the cab look inside and see if you can find the tube model and then ask here on the forum what monitors used that tube model. If you're lucky it will be a type that was used in televisions and your search for a new tube will be much easier as you could swap one out of an old tv.
Good luck with this.