Go to your local McDonald's.
Order a Double Cheese burger and small order of fries off of their Dollar Menu.
Ask that they put the double cheese burger on a Quarter Pounder bun (with sesame seeds on top). This substitution is free, because they are the same size.
Ask that they use the larger, fresh onions on your burger instead of the tiny rehydrated onions that they use on the small cheese burgers, and ask them to add some shredded lettuce. These substitutions are also free.
Ask them to hold the mustard and ketchup, but add Big Mac sauce. Big Mac sauce is a free condiment, just like their mustard and ketchup are.
Pay your $1.98 (plus tax).
When you sit down to eat, you'll have an almost-complete Big Mac (for a buck). The only thing missing will be that center bun. Construct that with half of your small order of fries (just to put a nice carb buffer between your two meat patties).
It's amazing how much seemingly useless things you can find out on the Internet.