Some of the Gen1 MBUS will be on sale now that the Gen2 models are out, but pricing should be pretty consistent across the board. Many on-line retailers will advertise a price that may be $5-$10 less than a competitor, but they charge you an extra $5-$10 in shipping. It's just their little way of making sure they show up with the lowest price on your search engine. The price may fluctuate depending on the color as well. FDE, OD, and Foliage Green MBUS will sometimes be a few bucks cheaper than the black.
Check out JoeBob Outfitters. They are an excellent retailer and I buy about 90% of my Magpul gear from them. They have the Gen2 MBUS for $35.95 front / $54.95 rear with free shipping...
JoeBob Outfitters: Magpul Sights