Ok by far the best deal that you can continually get with a meal is the infamous Whopper survey. Why do I say this? Because I am in one write now, abusing their Wi-fi for a chance to write to the reader and give it a go at making something interesting to take away.
So far I have yet to find a deal that tops Burger King's free whopper survey. It will usually begin with "Hello! Welcome to the Burger King guest satisfaction survey. Hello! I'm Jennifer." It really does do that. Then it will ask you to rate different food from 1-5, 1 being utter garbage and 5 being food from the gods. Usually you can get away with lying on the survey and pressing the number on your phone multiple times, but it is best to switch it up. If you answer the same way on the same phone, the location of the phone will be recorded and you will have to do guest satisfaction surveys until the "King" comes home. (Creepy guy by the way.) If you order fries and soda off the dollar menu, that is more than enough to make due for the rest of the week. Because you can do the survey EVERYDAY!!!
Ok but if you want to eat a day's fast food for under $10, the best way to do it is to have three courses of BK goodness. Note: I am not endorsing it as a health practice and I am typing this to essentially cover my own arse. Unfortunately Wi-Fi is only available at select locations. Total amount spent: $2*3 meals= $6 for the whole day. Heck grocery stores have a hard time competing at those prices.