How much do you think it would cost to make a catfish pond that is around half an acre in size?

Way to many variables. It depends on how the land lays. If you have a water supply. If you can push enough dirt to make the dam.
Their is alot of things that will make or break a pond. One bit of advice don't let some one use logs and trash to build the dam. Best way is a yard pan with a good operator.

I made mine a little bigger than 1/2 acre but not quite 3/4. It's 10 ft deep (shot with transit) in the deep side. I got 68 dollars in fuel in the tractor and 75 in drainage pipes. Dude was gonna charge me 8-10,000 bucks for a acre pond 8 ft deep. I said to heck with that and went to digging with the tractor. Got about 50-55 hours in it. Just working after work and on weekends. That was 5-6 years ago. Son has caught 5lb cat and a 5 lb bass this year. 3/4 lb bream all in it.