It has Nothing to do with not being able to afford it...Really? Lol. I dont understand peoples issues with being honest and paying for what you should. If you cant afford it, dont go.
Lets not get the thread locked with discussions of going around Disney rules. Abusing Disney policies will just cause more rules and regulations for guests.
Also, if they were to find out the OP could be asked to leave or pay for suitable accomodations.

But it is an American family trip tradition to have more folks in a room that is supposed to be....its not new.
Gosh i can't tell you how many folks add a person or two to a hotel room that says only 4. Not just at WDW but anywhere.
Its just reality
Ever been to Vegas?