I supplement my lunches with the instant cups. I get hot water at work so it's effort-free. I think they are less than fifty cents each. I will sometimes just have that for lunch. Even cutting back to sub-500-calorie lunches, I'm still having weight-gain problems. It's pretty depressing how little I can eat and still gain weight.
As Mike mentioned, "starvation mode" may be an issue, though I've found it takes a lot to get there. In early 2005, when I decided to get down from 225 to $healthybodyweight, I went on a 1200-1500cal/day diet. I counted calories religiously, but did not omit any specific food items except sugary stuff (candy, etc). I steadily lost weight at a rate of 1.5-2 pounds per week until I got down to 185, where I backed off and used healthy portions to remain in the 185-190 range for 5 years. I lost that 40lbs without any special exercise program except for an evening walk with the family, however I got back into cycling in 2006, which helped me maintain my weight. A job change and added stress, along with less portion control, saw me rise to 195lbs, but I've since brought that down to under 185. I've learned exercise alone will do nothing if you don't change your eating habits. I burn roughly 1k calories in an hour to two hours of exercise (running, biking, etc). You can kill 1k calories in a single meal without feeling full.
Anyway, if you haven't yet, get a physical and know your numbers so you can plan accordingly.
Your problem may not be calories, it could be blood sugar related. If you're prediabetic (BS of 100-115ish or A1C 5.7-6.3), you may need to get that under control first. FWIW, I'm prediabetic and mostly controlled it through diet and exercise until this year when my numbers rose to BS100+ and A1C6.3. Diabetes is common in my family, so it was just a matter of time. :( Metformin suppresses my appetite, so along with a low carb diet, I'm losing weight again.
Chris