It's always a nice thought to aim for a low-budget build, but it seldom turns out that way. Using (reusing) cast iron heads seems like a cheaper route to take, but unless you have a primo set of heads laying around, I would call your local machinist and ask what he would charge to 1. Pressure test/magnaflux 2. Mill deck surfaces 3. Do a valve job 4. Replace guides 5. Install seats 6. Price for valves
When you start adding up the cost of finding a set of cores, making sure they're good, buying valves, paying for machine work, etc., you'd probably be equal to or over the price of a new set of Edelbrocks/BBM's/Survival heads. Even a set of Edelbrocks out of the box would most likely net better performance for equal or less money.