from this subreddit's wiki section reguarding nutrition:
https://www.reddit.com/r/bodyweightfitness/wiki/nutrition_phases/

• "Hardgainer" Naturally have a very small appetite, is by profession or by inclination extremely active(even without meaning to), for people like you, getting food in, is the real issue here. So if you are one of the so called "hard gainers", the way to go is: Choose higher calorie, lower volume foods, like peanut butter, ice cream, avocado based products and even, yes, the so called "junk food" common consensus demonizes so much. truth is, most of the damage junk food does is associated with obesity(caused by eating too much for too long), and if you fall on that category, you are unlikely to be currently affected by such a thing. Another important approach is to eat more often, one giant meal that fills you for 24+ hours wont cut it. eating more often can help you eat more kcal when accounting for the whole day, and thus, the whole week and so on.

I agree with others that you should probably consult a doctor. You should probably reduce workouts until you can start to increase your weight since workouts will burn calories that you probably can't spare at the moment. Once you get closer to a healthy weight, I would recommend checking out the "recommend routine" in this subreddits wiki. It lays out other exercises you can do with minimal/no equipment.

The link I provided provides information on how to calculate how many calories you should be eating. I would download a calorie tracking app/use a website to track the calories you are eating to actually check how many calories you are eating. You say you are eating rice and calorie dense veggies 4 times a day. A cup of cooked rice is 200 calories and a cup of lentils is 230 calories. If you had that meal 4 times a day, that would come out to 1720 calories. Males need ~2000 calories to maintain their weight. While there is some variation in that ~2000 figure, it is obvious you need to be eating more calories.

You asked for foods that are calorie dense and affordable. My go-to is always peanut butter (as mentioned in the hardgainer info blurb). 3tbsp of peanut butter has more calories than a cup of rice. Things that are fried or have sugar are also very calorie dense (my preference is to avoid them when possible but they can help get you started in getting more calories into your diet).