Something like the Autopower you should be able to install yourself as a bolt in. If you or someone you know can weld the plates can be welded in. You'd be fine bolting in if you want to go that way. It's the least expensive though still what I would consider safe.
DOM has gone up quite a bit. I just bought three sticks of 1.75 x .120 today at $5.65/ft plus tax. That's up from $3.06/ft a year and a half ago. In that way the Autopower cages offer a good value at $500. To have the weld in kit installed by a fab shop it could be $2k-3k depending on how much of the car you have stripped. When I do a cage I drop the fuel tank and lines as well. If you don't have the tools (or the experience) there is no way to do it yourself for any less. Before I started cutting and bending my own tubing I got my first cage from https://rollcagecomponents.com/
. They've got an SCCA approved kit for about $700 plus shipping. For historic racing I think you'd be OK with an Autopower. I'd also do a 3 nozzle fire system and get a HANS or the like if they don't require it though most do now. Along with the suit, helmet and other gear you'll be about as protected as you can. You're more likely to catch on fire than flip your car when solo. It's never "safe". That's the nature of the sport but you can mitigate your risks pretty well.