Tansmission Out?

Ok, so everything was running nice, then a couple of days ago on my way home my transmission started to slip. It would slip when i started from a stop light and my rpm would go high without going anywhere, so when i got home i checked the fluid and there was slim to none in it so i filled it, that helped a little but still slipping. I took it for a drive again, and it worked fine about 80% of the time only slipping occasionally and erratically. I am assuming this means my trans. is gone or soon to be gone. Took it to a guy and said around 1300-1500 to fix does this sound right? Anyone have any ideas on how i could get one for cheaper, possibly rebuilt. Any help would be great.

1996 GT automatic conv.

It really was only slipping noticably for a day and it came out of nowhere it was working fine earlier that day, then i tried to drive it the day after like isaid and it slipped even worse. (You should probably know that it has 99,xxx miles on it)