last time i bought 1" it was about $45 a sheet, personally - i would not use it on this box, i would use 3/4 and resin the inside of the sealed chamber and then coat the entire outside, with the bracing i would use and the resin it would be more than enough (it would cost more to build the box with resin - but it would be air tight (and if you did not know 3/4 mdf is not air tight, it will allow air to travel through it, try the shop vac test.)), but i quoted it just as he asked, your price for $800 is about where i would be with all that work, i know that box would be $1000 at a normal shop, and they would not build it nearly as strong, no corner braces, x braces or most dont even router the port edges.