It could be $50-100+ for a 2400x1200 sheet depending on the thickness, grade and if you want it treated. Any H3 treated ply will be fine, doesn't need to be marine grade. DD grade is probably all you need, DD is rough on both sides, CD means smoothed for painting on one side, rough on the other. Try Carters for prices, and ask for a 'Mates Rates Card' first as it gets you 15% off retail.
You could build a steel or timber frame, then line it with treated ply for the structural support, then use thin sheets of perspex to line it and silicone the joins for water tightness. Would be cheaper than building a tank from big sheets of perspex/acrylic, and easier as the joins only need to be water tight, not structural, as the weight is supported by the frame and ply.
Another option is to fiberglass inside the ply then seal it with an aquarium safe sealant, or you might even be able to use plastic pond liner.