The way I see it the highest possible amount you could spend is something like this:
$2010 max on actual car and parts
$500 max replacing OEM brake parts ("Rubber or steel brake lines, master cylinders, rotors, drums, brake cylinders, calipers and brake pad linings may be replaced with new stock pieces." - Rules)
$1500 max on a pro-built bolt-in cage (if you went this route - it is legal in the rules, but most people build their own)
$150 max for new fancy harness
$200 on other allowed safety equip if you choose (fire system, window nets, etc...)
$500 on spare parts
$300 on entry fees (car + 2 additional drivers/banquet tickets)
Sub Total = ~ $5500
You should also expect to spend some money on tools that you don't have and supplies like saw blades, cut-off wheels, sandpaper, whatever.
Depending on how well you're already set up, you could spend almost nothing on this, or you could spend many hundreds of dollars.
Max cost of consumables/new tools: $1000 ?
Cost of travel is going to be your gas, hotel, and any vehicle rental. I suppose you could spend a ton of money here, but I think $1000 is a reasonable maximum even for folks going cross-country.
Total = ~$7500
That said I don't think anyone actually spends anywhere near this much, but that's about the highest you possibly could go within the limits of the rules and reason. If you want the bare minimum of safety equipment, and build your own cage/don't need a cage, if you have a good shop to start with and can travel cheap then you'd probably be looking at around $3000-$3500 start to finish, all expenses included for a very fast car. You can always spend less though.
I think at Wreck Racing we might spend closer to $4500 on a first-year car because our travel costs are high (bringing down a ton of students, and we go through a lot of consumable items, and need a cage since we run a convertible). However, once you build the car, the year-to-year changes are pretty minimal in cost, so you can keep changing the car and going back for only a few hundred.