Which is correct, "sales price" or "sale price"?

Acoording to Ngram they are both common definitions, but have different meaning as shown below. In your specific case 'sales price' is the correct expression if you are referring to regular, non-discount sales.

Sale price :

• The discounted price of an item from the regular selling price.

Sales price:

• Alternative term for price.

Source: http://www.businessdictionary.com

Although both are possibly interchangeable:

Sale Price is usually used when the item has a lower price than normal due to a sale.

The discounted price of an item from the regular selling price.

While sales price is an "alternative term for price" according to Business Dictionary.

In the retail industry we avoid this ambiguity - the common substitute is the selling price meaning the price it finally sells at.

Other terms include the cost price which is usually the price the retailer paid for it, the gross price which is usually the price before discount and the nett price which can be used as the price after discounting but before tax.

There are a few cases where sale vs sales could be either different, complementary or similar.

1. Different
   An equipment company could offer customers the choice of either buying, leasing or renting a piece of equipment. Hence sale price, lease price or rental price.

2. Complementary
   An equipment company would have engineering, manufacturing, marketing and sales department. If each of them are to be asked to propose a price on a piece of equipment, the prices would be engineering's sale price, manufacturing's sake price, marketing's sale price and sales' sale price. But internally, they would see no harm in labeling the various proposals as engineering sale price, manufacturing sale price, marketing sale price and sales sale price

   The equipment company may decide to have a week of sales. They would then have sales period rental price, sales period lease price and sales period sale price on a piece of equipment. They may use a shorter version by saying sales lease price, sales rental price and sales sale price.

3. Coincidental
   A store may decide to have a week of sales, meaning each item would be on sale. To refer to the price of an item, clerks could choose to refer to the store-wide situation of having sales. So that a customer could ask

   • What is the store-wide-sales price of this item?
     Which is a mouthful, and would rather ask thus,
     What is the sales price of this item?
   • What is the on-sale price of this item?
     but may choose to abbreviate her question
     What is the sale price of this item?