You can check out www.tirerack.com for prices on tires.  They're usually pretty good, but you will have to take them somewhere to be mounted, and that could cost a bit.  You do need to stay with run-flats, bdcause if you do get a flat, you can drive with no air for 50 miles at about 55mph before it will shred.  On the other hand, regular tires may be cheaper, but if you get a flat, say, on the highway, you will shred it in a heartbeat, you're out 1 more tire, and you have to pay for a tow truck.
    Since the originals wore out so soon, it was probably due to a lack of tire rotations, which should be done about every 5000 to 6000 miles, low tire pressures, or the vehicle is out of alignment, which is almost always true of new cars.  Also, regular tires are not as stiff, so the ride and handling characteristics of the van will certainly change, mostly feeling softer, but with less steering response and cornering stability.

You don't 'have' to stick with run-flats. You can put regular tires on your van and have no problems at all, however like the person before me said, you won't have a spare. You can remedy this by just buying one. There are kits you can buy that will let you mount it inside the back of your van, some of them come with covers so that you don't have to look at an ugly tire all the time.

I would go to the dealer I heard that toyota is paying for the tires becouse they are wearing out so fast

I have 2004 Sienna XLE AWD. Already got the check from Toyota 2 years ago regarding Tires.
Anyhow, Only disadvantage having Non-RFT on AWD Sienna is that you have to carry a spare in the trunk? Did I get it right?
If I want to switch from RFT to Non-RFT, Do I have to buy new rims as well? or is it ok to keep the current rims, and have non-rft mounted? <== This is my major concern. Thanks alot!