Is your customer willing to pay $50 more to get American made, high quality hinges and post caps on their vinyl fence? $100? You might be surprised at the answer you get when you ask. Offering an upgrade to your standard line of products doesn't need to sound like an admission of guilt. Your bid wasn't designed as a bait and switch, it is an attempt to offer a fair comparison with the bids your customer got from other companies.
The advertised price on a new car is a base model, just like your fence bid. Point out to your customer that you also offer a higher quality product with a lifetime warranty and superior appearance and function. If the bid is typical, it will be in the $2500 to $4000 range. If you offer to upgrade for $100, it will cost you $15 to $50 more for product, depending on the job, but that's $50 to $85 profit in your pocket, plus you have a better chance of landing the job, because you differentiated yourself from all your competitors.
Try it, and when you land that job with upgrades, call me, and I'll hook you up with the best caps, hinges, latches and drop rods in the business!
