I have a 1980 Pontiac Firebird that I bought for the low and it has a 350 small block chevy. I've Had it for a year. It runs great but now I notice that the battery is being drained sometimes overnight and sometimes within a month or 2.

How much do you think it would cost to fix at the mechanics. I live in south Florida, just saying if price may vary.

You can pull the battery out and take it to any AutoZone, Orielly's ect and have the battery checked 1st.  If you do have to take it in, it's best to eliminate as much as possible. Look for lights that stay on, like the dome light.