Actually, I think this is a reasonable price. He gave you a range of $800-$1200, and this type of job does depend on what is found after things are opened up. With a seized compressor, debris (shavings, particles) may have been blown throughout the system. Again, nobody knows until the system is opened up.
Sadly, yours is a familiar tale. All too often, a non-working a/c is the main reason people give up their cars and buy a new one. When the a/c quits, people don't want to sink the big bucks their mechanic quotes to get it working again.....not worth it on an old car. But they do want a working a/c, so they go out and buy a new car, even though their old car is otherwise working okay.
Frankly, I think fixing automotive a/c systems is one of the most financially rewarding jobs a do-it-yourselfer can do.
95 E320 Cabriolet, 131K