Add Hidden Unadvertised Sale Prices to Cart

I am looking for a code or a Plugin that will allow me to create a secret hidden price only viewable in the cart or at checkout. what i want to do is have it so you select X product that is $100 and when you get to cart or check out it becomes $75 without a coupon or changing the price.

One of my clients is a distributor of health products that other distributors sell as well. she wants to undercut them without changing the price Associated with the GTIN or MPN. Some of her competitors are doing this on non wordpress sites.

All i want is either this or some sort of secret way to advertise a discount only when something is in the cart