Gold is sold by the ounce... and it will depend on how long he would like the chain! I would suggest taking the broken chain with you and seeing what it would cost to repair the chain.
Last spring the chain on my husband's cross broke and he was devastated b/c it was given to him by his late grandmother. I suggested we go buy a new chain... no concept that it would cost us a fortune to replace the chain! It was well over $100. The jeweler was the same man who did our wedding rings and well-respected in the community so it was a valid quote. He asked to see the broken chain. With some soddering and additions, he was able to fix the broken chain for $25.
Just a thought!