What kinds of items do you want to store up there? Do they need to be secure?Protected from rain or sun? Do you have storage at home for a basket or box?
Soft bags are cheapest for the volume, but look for one that has a rigid base. Ones that are designed to fit between the rails on a minivan roof won't work on the Element's roof rack.
Baskets can range from the less expensive Rola (and likes), about 4" deep, to the Yakima and Thule ones (6" deep, with the option of adding bike racks on top).
Boxes range from 7-8 cubic foot ones like the Thule Sidekick (under $200) to large and expensive ones.
A bag or basket is fine for just driving to Grandma's, when you won't leave the contents on the car for any length of time (not even over night at a motel). A closed box is more aerodynamic, and provides more protection.
Here is an option that I wish I would have taken advantage of-
A local bicycle store that sells both Yakima and Thule racks and accessories has a Yakima locking cargo box that they rent out. This would have been more convenient as you wouldn't have to find a place to store your cargo box when you weren't using it. Check the Bike shops in your area
sixstring: nice setup! tires/rack everything! (pssst... specs on those treads?! ;-) )
As for the roof rack itself, Thule sells a bunch of different options. Cargo boxes (in a bunch of shapes/sizes), and at one point, they sold a rail-like platform (with no 'basket' per se). Always been a fan of the Thules stuff....
Keep a search open on eBay- I just got a Rola cargo basket, practically new with cargo net for $136.00, shipped- and from another seller a practically new Rola Platypus cargo bag for $45.00 shipped.....never pay retail. bubbeh!