Material that is cheap, an electrical insulator, withstands high temperatures, and will not outgas in a vacuum?

What first came to mind was glass that can be machined with carbide cutters, drills and grinding bits and is available in a wide variety of sizes, shapes and thicknesses at hobby stores. It is brittle of course and that may pose a problem. You could use epoxy resin, with or without glass fiber, depending on the strength you need. It can be molded into about any shape and makes a very high resistance solid when it cures and should perform O.K. at the temperatures you require. If you just need to isolate two parts, you can use an insulating varnish (available at hobby electrical suppliers) on paper. If you lay up multiple layers of the varnished paper, while wet, the composite will give you very good resistance, fair strength (much like thin fiber board), and relatively high temperature capability, probably good for at least 450 degrees.