Welcome to the forum!
I would guess between $25-40K depending on how much you bargain hunted and did yourself. If you sent it off to some popular bodyshop/restoration place it'll be well over $40K.
Just to give you an idea, I've had my Nova for almost 18 years now and it was in solid shape when I got it years ago, much better than the Challenger pictured above.
Here's a quick off the top of my head breakdown of what I've spent restoring my car over the years...
1.) $4K purchase price, engine/trans ran great, body solid, interior mint.
2.) $7K in engine/drivetrain upgrades (faster engine, better trans, converter, rear end, axles, frame ties, traction bars, drag radials, etc)
3.) $12K in bodywork, professionally restored, all new fenders, doors, quarter panels, bumpers, dash cowl, package tray, windshield, window felt, trim pieces, then put together and painted.
Without thinking hard that's about $25K and I started out with a solid car. Not to mention Chevy's are cheaper to rebuild and get parts for compared to Mopars, both older and modern ones from what I've seen.
If you want that car to have show car quality afterwards and a solid running fast engine/trans that will keep up with the modern cars like the new 392's and 5.0's then you'll easily surpass the $30K mark and probably get near the $40K mark or higher when it's all said and done. Certain Mopars are worth that or more when professionally restored, but I don't think a 1973 Challenger would be.
So what you're looking at is an immediate loss on investment in this car IMO. BUT, if the car has a lot of sentimental value AND you have the time/money/interest to do the car up right, then I'd say go for it. It's a hobby, that's how it goes. I knew when I sunk all that money into my Nova doing the bodywork and getting it painted professionally that I'd never get my money back out of it, but it was my first car, lots of memories with it, so I did it anyway and never plan to sell the car, so it really doesn't matter to me. If you have that attitude, then you're golden. If you think you can restore it and make a massive profit on it, you'll most likely be very disappointed.
Hope some of my rambling helps!