Every 10 pounds you lose in weight, you drop a size. And continuing to wear clothes 3 or 4 sizes too big will make you look and feel terrible. You’ll need to buy clothes from scratch, whether it's office wear, formal wear, jeans, or dresses, everything has to go. You'll need a couple of thousand dollars at least to replace all those, and if you continue to lose weight, you'll need a couple of thousand every 2 or 3 sizes down.
Now, I agree that losing weight is a challenge. But not putting on weight is also a challenge. The kind of sedentary lifestyles we lead, and the unhealthy foods we eat, gradually keep putting on us those extra pounds. Most of us need a lifestyle change, but it doesn't have to be a radical one. Sensible choice in food (fewer carbs, more fruits and vegetables) and moderate exercise could keep us from packing on those pounds, and prevent us from the costly health problems associated with obesity. Maintaining your weight is just one way to save money on clothes.
Of course, there are exceptions to every rule. Generally speaking, it makes more dollar sense to just maintain your weight, but if you are already obese, maintaining your weight could cost you more in health problems and medical bills.