This mod will allow you to enter a Manufacturer's Suggested Retail Price (MSRP) in your admin area on the product page.  If you enter a MSRP, then on the storefront, it will show the MSRP, Your Price and You Save in percentage.

MSRP: $100.00 (with strikethrough)
Our Price: $75.00
You Save: 25% ($25.00)

Versions 1.3.9, 1.5.0