Paver Base - Sand, Polypropylene or Other

I'm preparing to move forward with a patio paver project that has been a long time in the making - mostly because it was initiated by my wife on a shoestring budget (Shoestring Budget = No Budget).

We're paving an irregular area in our backyard that we've regraded and compacted with fill dirt to level, with the help of a retaining wall I built a couple of years ago, where there was previously a fairly considerable slope to the property. I previously received some good advice and counsel, especially regarding backfilling tree roots (although happily, the trees are all thriving [growing and/or still living] now after close to three years of being "backfilled").

We're now getting to the stage of contemplating the patio stone foundation, and we've been considering using the Brock paver base panels. Harkening back to the "shoestring budget" issue, I know that the foundation for patio stones is very important, and the Brock panels have a lot going for them that is attractive - the least of which is the $12.99 per panel cost. This material (@ 2' x 3' each) would likely translate into a cost nearly equivalent to the cost of the patio stones themselves.

Consequently, while I like the idea of the solid base over the level sand foundation to work from, I've been looking at some other less-mainstream alternatives. Since these Brock panels are nice mostly because of their ready-made flat surface and ease of cutting/working with, I began to wonder whether some sheets of high density, 1-inch insulation, might be suitable substitute for the 1/2-inch Brock paver panels. The insulation panels are not even in the same neighborhood (expensive neighborhood) as the Brock panels, and they are specified as highly moisture resistant, and at 1-inch in thickness, they are roughly the density of the Brock panels, and they are interlocking like the Brock panels too boot. I wondered whether the distributed load of the pavers (plus the eventual people) on the 1-inch insulation sheets would hold up reasonably equivalent to the Brock paver panels - since clearly the insulation panels were: 1) never intended to be installed in what will be a permanently damp environment and 2) were never intended to installed in a horizontal configuration, and with a load placed on them.

This is where the collective audience is invited to quickly (and emphatically) alert me to why this is many different variations of a bad idea (which is why I post stuff here in the first place, although getting the endorsement of the occasional "good idea" has been semi-rewarding also . . .). Thanks!