Never discount cheap if you can actually use it.
you can mount the standards on 2X6's 6' or 8' long at each end of the standard and slide them under the front buns and they will not tip over, plus they can be broken down and moved easily.
Soda crates from Coke tied together with wire ties 3 longways and 1 across makes a dandy skid to lift the mats off the ground and they don't rot. Plus they stack up to take up less space.
Plastic skid with solid tops work better, but are usually not free. ATV's and Motorcycles, and Snow toys type stuff are sometimes shipped on these. The point is, they dont rot or break easily.
You can get "minus" from the quarry, and tamp it in place or rent a tamper or roller and with a little water, you have near concrete strength unless you live near a tropical monsoon area. That's how roadways are built today, only on a grander scale. Remember, you have to remove the grass and dirt first to about 5-6" depth. If you do this really well, you can get bags of asphalt patch and cover it if you want to. But it's just additional cost. Be aware such an endeavor may bring up some issues with your homeowners policy.
If you REALLY want to use them, Make sure the extra "mattresses" are really mattresses and not BEDSPRINGS. You could probably wrap them in 6 mil plastic from a farm supply or Home Depot temporarily. Maybe waterbeds would be a better choice except for the spikes!

Be aware they all rot easily and often become a home to a lot of things you probably don't want near a home, smellwise, rodent-wise or otherwise. I recommend finding at least two older pits or a HJ and PV pit and putting the pieces together.
As Bruce said, there are a lot of older pieces floating around out there. The older Blazer stuff got replaced pretty quickly due to their original design, and a lot of schools still have them up stuck up in the rafters of their storage buildings. that's where I got mine.
One suggestion, if you can find an indoor soccer park that has the space, you might talk them into lettng you use a raised runway and pits on slack soccer days. And their facility probalbly already has the liability insurance that it needs to operate.
