The DiscountedPrice from REX, below is calculated from the pricing hierarchy in the following article How do I set sell prices for my eCommerce store? 

If the sell price (DiscountedPrice above) is less than the RRP the integration will set the sale price in Woo with the DiscountedPrice and the regular price will be set as the RRP. Your theme will then display the 'on sale' graphic.

For example this product below is currently on sale.

This discounted price can be over-ridden if you set a "Maximum Discount Rule" against the product. This feature can mean that you can not see the actual price on the product.

You have an outlet discount price applied to this product that does not comply with the maximum discount rule applied to this product so say this is set at 5% - this means REX will not set the sell price below 5% of the sell price which would calculate as 37.95 rather than the 24.95 you see below. 

See retail express support article ....