I bought new 2" 4x8 (32sf) sheets of XPS (again this is new) for $12 per sheet (less than $0.40/SF). So I'd say that's a royal ripoff but I do know that thicker is more expensive. However you can put two layers of 2" down for about $0.75/SF and it is new insulation. Your price for used is 33% more than that.

Oh and I'm in chicago and that was from Home Depot.

Surfsup,  how long ago was that purchase? What product was it? XPS 2" 4'x8'  is 3 times that in Maryland.

New XPS in my neighborhood runs 12-13 cents/R-ft^2. Used, damaged, or factory-blemished XPS runs about 3-5 cents/R-ft^2. At R15 & $1/ft^2 you're looking at 6.7 cents/R-ft, which considerably more than what it would cost in my local market. If it was 50-75 cents per square foot it would be "in the range". But there are multiple vendors within an hour's drive of my home or office- a true market with competition.

At 3" it was originally about R15, but in hit's UV-degraded state it's probably lost about R1 or more. In 50 years it'll be R12.5- R13, anyway, so that's not a big deal.

Thanks for chiming in...I'm going to go back and offer 25c sq/ft for the whole load 3000 sq/ft... if they say no they can keep it. Its pretty beat!

In this area (Western Arkansas, Eastern Oklahoma, SW Missouri), I routinely see people selling used XPS insulation from chicken house tear downs on Craigslist. The most common size is 4'x21'x1.5" thick (DOW Ag Board) and prices generally run $10-$15 per sheet ($0.12 to $0.18 per sq. ft) for R7.5 (new) material. The R value of this aged 1.5" XPS is probably around R6 now. The edges are usually pretty rough, but it would be good for below slab insulation.

Chased down another local source for foam insulation today.. now this is EPS not XPS and my understanding (please correct me if I'm wrong) is that the EPS is a better performer as the temp falls while XPS does not and... EPS is more vapour permeable which is needed in my retrofit over existing 2x6 with a 6 mil vapour barrier on the interior.

they have a variety of sizes all priced at 10c/sqft per inch so 2 inches is 20c a sqft etc..

I found 2, 3 and 4 inch thick EPS a mix of type 1 and type 2 (not sure if I should prefer one or the other) but in quantities that will easily look after my requirement (now I think its 5000 sqft at 4").

800 sqft under the new pad = 800
180 ft of perimeter wall 4 ft horizontal frost protection = 720 sqft
180 ft of perimeter wall 2 stories tall = 2880
totals 4400 so with a 10ish% fudge factor =5000.... 5000 x 4" @.10/inch $2000 = a BARGAIN

Before I move on I have to confirm...

is 4 inches EPS under the slab enough

is 4" on the exterior walls of both existing and new construction enough...

I know there is a rule of thumb about 2/3 of the insulation being outside the wall but with R20 in the wall I would need R40 on the outside...that seems unlikely??