After a few decades in the marine industry, I have a few thoughts on this:
1. Chain in the drum ships for free. If a vendor orders from Acco/Peerless, etc. the manufacturer will drop ship to US addresses at no cost.
2. The standard pack for 5/16" is 550' if memory serves. My guess is that if you can get a great deal on a drum of chain, you'd be able to sell the remainder at your cost or perhaps a profit.
3. There's a ton of margin in chain. What bothered me about my recent job was that they gave teeny discounts on long lengths of chain, and people who buy long lengths of chain are likely the company's best customers.
4. I would remind whomever is making the quote that they have to make ONE phone
call and pay ONE invoice to make this happen. No inventory carrying costs, no employees pulling 20' at a time in an expensive store. This is the ultimate example of a low marginal cost transaction. If I were an independent-thinking associate, I'd do this transaction for 15% GM, and buy your loyalty for some time to come.
So, my point is that I think you should get a few companies to bid on this, and consider buying
an entire drum. You might try to pre-sell the chain you don't need for your use. Obviously, this would make more sense if the buyer were in your marina, or at least your town...
One final point: although not windlass-friendly, I think a double clevis is about the strongest way to bend two lengths of chain together. It won't work
well in the windlass, but it might help if you need to pay out a long length of chain prior to a hurricane