Pinniped, I have some experience with longer-term stays (I have over 400 nights credit with Marriott this year alone), and I don't find anything unusual about this rate, assuming Eugene is correct.

You'll often find tiered rates that get lower the longer you stay. I can't speak for all properties, but the LT discounts usually kick in at 5 or 6 nights. A larger discount might kick in after 12-15 nights, and the largest discount of all generally appears after 30 nights' continuous occupancy. I've generally seen three or four sets of rates (varying with stay length).