“I have a ‘prospect’ who has plenty of the competitor’s products and continuously asks for quotations but never buys from us. The prospect keeps insisting that we have a chance to break into this account so Sales keeps accommodating them. We end up spending valuable time and effort doing these proposals for nothing. Should we continue to quote?”
Remember the saying “Fool me once, shame on you. Fool me again, shame on me”?
Stop quoting and explain the reasons to the "prospect". Many companies must solicit a number of bids before purchase. Worse yet, some buyers will simply use your quote to get better pricing from other sellers or their favored seller. If you see the pattern, share your reservations withe the "prospect" and explain why you will decline based on past experience. Inform him/her that if their purpose is solely to get another competing bid for purchase or leverage, you can still help them by providing a quote with a low offer - but just for that purpose. Once you call a spade a spade, some of these “prospects” become more reflective on their actions. I’ve had a few actually explain why the current situation is different and end up buying from us after one final opportunity.
Same applies to giving away free services such as consulting or testing. If you do not value your time and efforts, why should anyone else?
This reminds me of the girl who kept copying my homework and strung me along in elementary school. I finally got wise, cut my losses and moved on. Thanks for re-opening this old wound. (I kid. I kid.)