If you get Amazon emails, check for a coupon on Britax. Here's what I found in my email today:
Dear Amazon.com Customer,
Customers who have shown an interest in Baby items might like to save $5 with instant coupons on Britax Parkway Booster Car Seats and $10 on Britax Frontier 85 Combination Booster Car Seat, Canyon shipped and sold by Amazon.com. Just click on a coupon below, add the item to your cart, and the discount will be automatically applied when you check out. Offer valid through March 31, 2013.
Britax Marathon 70-G3 Convertible Car Seat, Onyx by Britax