Are they taxing you based on the fixed expat rate or are they putting you on the local tax rate? PM me your salary, position and package. I kinda know the market rate for expats and locals because my colleagues has shared a lot of info with me before. I can help with some intel if you want.

Yes, its good that they are covering your rent and a bit of the food. Those two are probably the biggest cost to your monthly expenses. Subway is considered affordable in KR because most stops are only 1250 KRW. Taxi is not too bad too with most rides under 15k to 20k KRW within Seoul, but rides in and out Seoul will be expensive.

Cost of a normal meal in Seoul is anywhere from 5k to 15k KRW. Three meals a day for 30 days is then about 450k to 1.35mil KRW. You can do your own math for your own food budget. This is all purely eating out. 

You can save a bit if you cook mostly Korean food and eat it for days, but you will get sick of it really fast. Imported stuff is expensive and not easy to get. To be honest, I find it cheaper and easier to eat out than to cook for one person.