Single people tend to eat a lot of fast food. Cooking at home for one is often more trouble than it is worth. Most recipes are designed to serve four or more people, and some of them don't scale very well. Food spoils, which makes it hard for the single person to get any sort of good deals on groceries. If I buy the value pack of 20 smoked sausages then I am going to have to eat smoked sausages ten times in the next two weeks. Sure you can buy smaller packages, but the smaller packages are priced so much higher per unit that the fast food starts being fairly competively priced. Plus, I find cooking meal after meal to eat by myself at home to be depressing.
I am quite frugal in most respects, yet even I tend to eat fast food an average of one time per day. But you can still eat fast food without breaking the bank. Just follow a few simple tips and tricks and you will be able to shave significant amounts of money from your monthly fast food expenditures. Sure you also have the option of not eating fast food. That is a great option for many people. But then there are people like me. I actually gain weight when I do all my own cooking, and I really don't save all that much money at home anyway. I track my expenditures, and my monthly food bill in a month where I avoid fast food is only about $100 less than it is in months where I eat it daily.
The first thing you can do to cut those costs down is simply to look for coupons. Fast food coupons are not as common as pizza or grocery coupons but they are out there. If you are lucky then you should be getting them in the mail, or finding them in the newspaper on a weekly basis. Most people just throw them away, then go to the exact same fast food place three days later and buy stuff without the coupons.
I have found that Burger King tends to have the absolute best coupons, while McDonalds usually seems to have the absolute worst ones. A typical Burger King coupon will give you a free $3 sandwich when you buy a value meal, while a typical McDonalds coupon will save you 50 cents on some esoteric item that you never would have purchased in the first place. Other chains seem to fall in between these two extremes.
Look out for in store promotions. Learn the promotions and take advantage of the promotions. When McDonalds has the Monopoly promotion you can get significant amounts of free food in the first week of the promotion. The cups, hash brown packets, french fry boxes and Big Mac cases with the game tickets on them come labeled for each week of the promotion. The week one items are heavily seeded with ones that win free food items. The ones for the later weeks have very few winners. I know this both from experience (working and eating at McDonalds) and from a scientific examination of two cases of unopened hashbrown sleeves, one from week one and one from week four. The week one case of 1000 sleeves had over 400 winners in it, including 4 winners for $5 worth of gift certificates. Meanwhile the week four box had less than 100 winners in it, and no gift certificates at all. How when and where the hash brown sleeves left McDonalds and made their way to me is a closely guarded secret (and the evidence is ten years gone by now anyway).
Different restaurants will often have promotions where you get a free value meal if you purchase gift certificates or gift cards. The single best promotion of this kind I ever saw was from McDonalds. At one point McDonalds was running a promotion where they would give you a free extra value meal with the purchase of $5 worth of gift certificates. This was back when all the value meals were $3. But still, that is a significant discount. It was even more significant because over half the time the cashiers would let me pay for my $5 in gift certificates (plus value meal) with the $5 worth of gift certificates I had purchased on my previous visit. I haven't seen a deal that good in a long time, but I do see free value meals with purchases of $20 or $25 gift cards on a regular basis. Just today I got $6 worth of free food at Jack in the Box for buying a $25 gift card. I can be darn certain I was going to be spending $25 at Jack in the Box in the future anyway, so buying the card was a no brainer.
Learn to complain. If they get your order wrong complain. If the food sucks complain. If your drive thru order is bad call and complain. This will get you free food as often as not. The more free food you get the less you have to buy.
Finally you should learn the various restaurants and locations. You should learn which ones to vist and when to visit them. For example, Jack in the Box has the best deals on breakfast, while McDonalds has the worst. The Burger Kings in my area are massively expensive at all hours and should be completely avoided unless you have a coupon. Prices at the same chain will often vary by location. I have two McDonalds the same distance from my house. One of the McDonalds has double cheeseburgers for 99 cents, while the other one charges $1.89 for the same sandwich. Which one to choose in that case is obvious.