I suppose there is no hard line in determining when a car is "too far gone" to bother restoring. I have seen show cars built from mere threads of original metal.
The answer lies more in "how much do you want THAT particular car"?
The problem with sinking tons of time and money into X1/9's is that even when fully restored, they simply don't have much market value. You will never be able to make the decision to resto an X1/9 based on what it will be worth to someone else when finished. Its about what is worth to YOU.
I recently saved a very rusted X from the wrecker. I kept good records on everything spent. I did all the bodywork, welding, metal repair, electric, interior, mechanical, etc. myself and only sent it out for final paint (and I got a good deal on that as well). All told, I actually believe I could sell the car for more money that I spent. I probably have around $4,250 in hard costs and I suspect the car would sell for over $8,000 (its pretty perfect).
However, when I factor in the cost of my labor even at a minimal $20 an hour, I'd have to get around $58,000 to break even (2 years X 25 hours per week X 52 weeks X $20 per hour = $52,000)!
The point is, I would never sell that car - I have way to much physically and emotionally invested in bringing it back to life. Had I spent that time on restoring a 67 Mustang, it might be worth ten times as much.
The good news is, if you just want an X because you them, there are plenty for sale for under $3,000 that are probably in much better condition rust-wise than yours. You could chop yours up for parts, sell them and recoup much of the cost of another car. If however, you love that particular X, that its just a question of how much you love it.
Ed