You should ask "How big of a lake do they need?" Even if they only attain half their possible adult size, you're still looking at a 300lb catfish.
And, the few times I've seen them available, they were going for triple digits wholesale. There's a mass of imported paroon sharks hitting the US right now. That's most likely what you've found. These cats grow to a slightly more manageable 3 foot size. A proper tank for these fast swimming, High O2 demanding cats would be in the 10ft x 4ft x 3ft range.