If your A/C system isn't working because the r-12 leaked out, there are a few things you will need to do before you can add any R-12, and even more before you can add r-134A
If you decide to add r-12, you NEED to find the leaks, otherwise your expensive r-12 may last the rest of the summer, but won't be there come winter time when the o=rings shrink due to cold weather.
If you want to convert to r-134A, then you need to change the o-rings anyway. There are different o-rings they use for 134 so you will need to swap yours out for the 134 rings.
THEN, since your system has been drained, it's more than likely regular atmosphere and moisture have found there way in. You will need to pull a sustained vacuum on the system before you can add any refrigerant.
Another step that you *should* do if converting to r-134A is replace your condensor with a r-134 unit out of a 94-95 Mustang. You technically are supposed to change your compressor too, but now things are getting expensive. The orifice tube also needs to be replaced with one from a 94-95 Mustang.
I forget the calculations, but i beleive you are also supposed to add either a certain percentage more or less of r-134 than the system calls for r-12. It's been posted before. You also need to flush your compressor of it's old fluid and add new fluid before you pull a vacuum and add the 134
There are some good write-ups on here about correctly converting a car to R-134A. Like said before, to properly convert is going to cost a lot more than a $40 autozone kit.
If you just buy a can and hook it up, it may be cool, but it sure as hell won't last.
And i definitely wouldn't toss r-12 into a system that has the potential for leaks. If your A/C hasn't worked in a long time, there's a good chance it will be empty again in a couple months.
This is why fixing A/C systems is expensive. Sometimes there is a lot more involved than just hooking up the refrigerant and pumping it in. $1000 for a fix sounds reasonable to me given the potential of work that may need to be done to fix it correctly. If you want to go slap a $15 can of r-134a in and call it a day, be my guest.