We should stay more than three more days with that card. If we don't, it won't be worth it. It is as if it is a "ten-day card" (ten or any figure greater than 3) and one is saying that it is too expensive to be used for a three-day stay. It is as if the card could be used for other things and we have a choice.
Don't stay three more days with that card. That'll come out too expensive.
I agree. Only context can make it clear.

As for the 'too dull to/for', I would say:
- This knife is too dull to cut meat.
- This knife is too sharp for a 4-year-old.
I reckon you can say "too dull for cutting meat", but I still prefer "too dull to", though "It's made for cutting meat" is fine.