Need a cost effective foundation for our new spa. Concrete (expensive) shell, pavers, extend trek deck... suggestions please thanks

Are you installing on ground level or what?

Above ground. We have a beautiful trek deck and I was considering extending it. But then thought why? The spa would cover so much of it. Then considered concrete, but that is costly. Now wondering about the hadi pads?

How high off the ground is the floor of your exiting deck?

It would be a step up from the existing deck. 8 inches or so from ground level.

So if I am picturing this correctly you could set the spa on the ground and its top edghe would be about 2' above the current deck surface, right?

no... (sorry I am not being clear) it would be on top of deck.

We just installed a spa with Handi-Pads set on 4 inches of decomposed granite. So far (6 weeks later) it looks & works great. Be sure to use the decomposed granite, not pea gravel. The granite is irregular shaped, pea sized, and locks into place. You get perfect drainage and an easily leveled surface for the Handi-Pads, which will ensure your spa's warranty isn't voided if you get a new one.

I suggest taking a look at EZ Pads
For your project.