It can be difficult to be gacious when you lose a sale, or a contract for services, especially if it is something you desperately needed. "Grace under fire" may be a cliche but it is also an important term that describes the kind of response you need to give in such circumstances. It is, after all, truly a small world these days. The Internet gives your business global scope and, at the risk of quoting yet another cliche, "What goes around comes around!"
You want to respond positively, and with grace, to a rejection. You never know who the lost customer will speak to. I've often lost a sale for one reason or another, often not even knowing why, but then, not long afterwards I make another to a customer who tells me they were referred by the same customer I lost. My handling of that lost sale was enough to get me a referral. That's not bad at all!
Okay, the last cliche: "Don't burn your bridges!" You never know what good will come from a missed oportunity. Being harsh, or just not responding at all, will not lead to positive outcomes. But thanking the lost customer for their time, telling them you appreciated the opportunity to quote on the project, service or product need, leaving on a positive note, can lead to unexpected future success.