I would stick with the 80-82 sohc carbs. The 77-79 ones have issues with pilot mix screws breaking off in the bodies. I would try to find ones that are clean and just need some freshening up. Prices and conditions will very greatly. I have sold cleaned and re-built set from $125-$185 shipped in the past. I have seen very dirty sets that need a full rebuild go for under $50. But time to clean and replace broken parts can run $100+
With your bike you need to keep in mind that your going to spend money on it. It will need new tires, chain and sprockets, battery, carbs/intake parts, plugs wires, brake work and lots more. I have bought bikes for $200 but still had to put $500+ into them to make them safe and reliable. That's just for parts as I do all the work myself.