Right now Amazon is offering Just Dance 4 for $19.99 plus free super saver shipping or free 2-day shipping with Amazon prime membership. This price is valid on Wii, PS3 and Xbox360 platforms. Normally $39.99 this is the lowest price I’ve seen so far.
Remember, Amazon pricing can change without warning so don’t wait long. Just as a reference Walmart.com is selling them for Wii and XBOX360 for $39.99 as well with free in-store pickup and free shipping with $45 purchase. If you go through Amazon you don’t have to leave the house!