How much does it cost to have a roll cage built?
Like most things in the world, there's no upper limit if you want to get technical. You can buy a Rolex watch for $10,000, right? And surely there's watches even more expensive than that. But what is typically needed?
Generally speaking, expect to pay $2,500 to $4,000 for a basic custom rally roll cage and seat mounts.
The same guy who is doing the cage work might do other work. Removing sound deadening material is one example, you can do that yourself with some dry ice from the super market. Seam welding (which we don't recommend for new drivers) is an example of an additional service which they might offer or recommend which would increase the price. Reinforcing suspension pieces, fabricating mounts for this and that... a wise man said once "There's no limit to what you can do with a big spool of welding wire." But that doesn't mean that you need those things to take the start line at your first rally.
So can you pay $10,000 for a cage and shell prep work? Sure. And that's not the upper limit, either. But you don't need that to get started.