Www.vintagecardprices.com is an invaluable resource for recent sales prices. It's well worth the monthly subscription if you plan to do some buying and selling, especially if you're new to the market. Looks like the average recent sales price for a PSA 5 '51 Bowman Ford is about $592. Even the higher-end sales are well under $775. Sometimes it can help to show the seller the info so he knows you're not lowballing. If he won't come down, there will be plenty of more opportunities. Good luck!
Well let me add one caveat - if you love the card and will enjoy it no matter what, then that's more important than the "average sales price." But try to save a few dollars anyway!