They're not cheap! I've had a Nauta 70 ltr flexible for three years now and it's provided excellent service. I use it as an additional tank lashed down on the foredeck, for long runs (Scillies etc) then fold it up when I get there and put it away.
They are well made and because they are certified for both marine and aviation use, they are, I guess, hi-spec, hence the high price. (Show me anything to do with aircraft parts that's cheap and I'll bet it's a fake).
Anyway, don't want to steal Ribshop's thunder but I would expect to pay in the low hundreds