So as you can see all of those sheet good run from $2.00 to $2.25 per square foot. I think all of them, even the expanded PVC, are better choices than thin acrylic.
All of them can be solvent welded and all of them can have the corners reinforced with some sort of solid angle or bar stock.
Of course when comparing a sheet membrane to a liquid membrane you have to factor in waste. Generally speaking liquid membranes have about 10% waste when applied to a smooth surface. As the surface texture increases your waste increased but you end up with a stronger mechanical bond.
With solid sheet goods you could possibly have zero waste but even then you'd have the added cost of the adhesive to adhere the material to the plywood substrate. If your dimensions weren't some convenient multiple of 48 x 96 or 40 x 72 in the case of the styrene then you could end up with a lot of waste.
For reference, you can waterproof a smooth plywood tank with a epoxy at 10 one-thousandths of inch, or ten mils (not to be confused with millimeters). That's not accounting for waste and what you'll lose to a textured surface so let's add another ten mils, or 20 mils total. Epoxy resin is 100% solids so when it's cured it has the same thickness as it's wet coating.
So now let's picture a 48 x 96" mold that is 20 thousandths of an inch deep. That's 92 cubic inches (48 x 96 x 0.02). There are 231 cubic inches of solids in a gallon of epoxy so it would take 0.40 of a gallon to waterproof a 4 x 8 sheet of plywood.
The thin epoxy resin from US Composites has been used on a MFK build and is less than $50 per gallon depending on volume purchased. So it should take about $20 of epoxy to water proof a sheet of plywood.
So as you can see the 3 mm plastic sheets you're considering could be considerably more expensive, especially if you can't use the sheets fully. But with the plastic you're getting 125 mils of protection vs. the less than 20 mils with epoxy.
And with epoxy you have the option of adding fiberglass reinforcement to the corners or to the entire panel. Now that adds cost for the fiberglass and for extra resin to wet it out. There is information on the web about how much additional resin is needed to wet out certain fiberglass materials. Years ago the rule of thumb was the additional cost of cloth over mat and roving over cloth was a wash because mat takes the most resin to wet out, then cloth, then roving. So it was better to use cloth, for example, because it was stronger and easier to work with than mat. I don't know to what degree this is still considered true.
To briefly touch on Drylok and the Insl-X chlorinated rubber pool paint, both are about 50% solids so when you buy a gallon of it you only end up with half a gallon of cured material once all the water and other liquids evaporate. And given they need a dry film thickness at least as thick as epoxy (and up to three times as thick for some rubber products) they really end up being more expensive because they require six times as much product (three times the thickness x 1/2 the solids equals six).
Now these numbers don't always add up to how the products are used in real world situations. But it's a start if you're comparing costs.
So before I ramble on too much more, I want to say again I like the idea of a solid sheet material used to line a tank. But unless your tank dimensions cater to it the waste can be really bad. And you really need to beef up the corners. You can either solvent weld more solid material over the corners or find a caulk that makes a good seal with it. I can tell you that silicone rarely bonds as well to plastics as it does to glass.
I also like the idea of two different layers of protection, like dryloc over expanded PVC, assuming good bond strength. But that really adds a lot to the cost.
Hope this gives your some food for thought and doesn't raise as many questions as it answers. I hate that, lol.
