At subway: the food is fresh, you don't have to make an extra trip, and it's made for you. The nuisance may be that you wait 5 min/day in line. Perhaps you can argue that food poisoning is more of a risk at subway. You could also get tired of the food. Looking at things outside of costs for homemade, you have to physically go and shop, take 20-40 minutes out of your day to shop, risk an accident while driving,and spend time preparing your meal while creating more dishes to be washed. Also, you may have to pack your food for the day before going to school. However, you do have more of a range of food by buying your own and cooking.
For costs at subway, let's assume you buy 1 sandwich per day from subway (5 days a week). That's $35-$40/week. $140-$180/month. For homemade costs 5 days a week (1 meal/day): In order to have fresh food, you will have to buy lettuce, tomatoes, bread, and meats, etc each week. Let's say $25/wk & $100/month for those items. After initially buying things that you only need to buy once a month like mayo, mustard, etc, you will spend roughly $10/month for 1 trip/month items. You will also use gas:let's assume that trip costs $2.00 in gas. That's $8/month. You also have to cook your food and clean dishes let's assume power bill is $6/month more.
TOTAL costs for subway (5 meals/wk 20 meals/month): $35-$40/wk = $140-180/month
TOTAL costs for homemade meals to replace subway meals (5 meals/wk 20 meals/month): $124/month
CONVENIENCE FACTOR subway = a lot better than homemade meals
Conclusion: Subway costs more and may get old, but is way less of a nuisance and the food is better (my opinion)
Solutions: In order to make grocery shopping less costly, buy all 15-21 meals per week at the grocery store and spend time finding coupons. You would also have an easier time buying in bulk and not wasting food bought at the store which would lower amount needed/month and save money. This would also create more time spent preparing and planning you shopping and cooking.
This was actually fun, good question. Hope this helps. It helped me at least!