Most of the time in sales, why to quote a different price for the same product to different customers?

Quotation different price for a same product to different customer are occured due to the following reasons :

1- Place & Environment customers/ differnt  conditions & markets 

2- Nature of competition in the market which is differnt from one to other

3- Demand volume in the market which is different from one to other

4- Level of the risk incuurred in the payment, regular or irregular client

5- Cost of the offering as per the customer request / private request

6- Level of the business with customer, long term & Size of quatities

Sales is all about profit. It heavily depends on client relationship, long term business and quantity of order unless you are a brand like Apple, Microsoft, Intel etc. If it is a one time business we wouldn't want to give any discount or promotional offer but for a regular client, we would give a lesser quote as it brings a small profit but has a long term business compensating for the loss on single delivery. 

I fully agree with Mr. Sattar Abdulkarim's excellent and insightful submission. Thanx for the invitation

Dear, in simple term, it all depends the market. Second the landed cost. Plus again depends on the customers repeat orders. Also the demand of it. Again it depends - is it one piece, 100 PC's or container load. Above all again it matters is it spot cash deal, or 30 days, 60 /90 days, 180 days. Case to case customers are dealt depending loyalty, feeling long term partnership goals, or one time deals just for your product there is demand in market, so client takes and when no demand he doesn't care, so case to case, keeping overall situations making things viable by the seller.

we cant keep our price fixed for all customers as per the customer payment terms, quantity and frequent buyer we change our price.

The question might not be an academic one, it still is an interesting one. In common trade it is generally accepted that it is something that can not be done. We all know how fast that kind of information is shared between other customers and competitors alike. I assume that you are talking about the same product, so I will exclude two obvious exceptions that would allow different pricing to the same customer segment: difference in quality and the possibility to sell exactly the same product under two different brand names. I would also exclude de use of exactly the same price structure but used within the context of different side arrangements. For instance advertising contributions, return policies that are converted in straight discount percentages, volume discounts etc. are changing de facto the price structure, but they often are well hidden and should not be considered as a difference in pricing. The only successful use of real differences  in price structure that I know of is the use of the brand as leverage to obtain higher prices from for instance discount stores with the purpose of obtaining very similar consumer retail prices. It allows the manufacturer / distributor / importer etc. to offer the product both to a regular retailer with full mark-up and to a discount store with substantially lower mark-up. Contrary to expectation, it is the discount store that will be offered the higher price. The few cases I have seen in which this works well is when the brand is so strong that the discount store wants the product even at a premium price.