1973 Chevrolet Nova Cost of Replacing Timing Chain?

My 73 Nova v8 307 stalled as I was on an exit ramp with a rattling noise. I tried to start it again, (forgetting that this happened to a previous Nova) but it just rattled and would not turn over. I'm pretty sure the timing chain broke and hopefully I didn't damage the rods. I'm waiting to hear. But assuming it's just the chain, how much should that cost to fix?