How are your monthly costs broken down?

$500/month

Tithing: $50/mo
Because it's Biblical, and I want to continue to be obedient in this area.
Food/Water: $100/mo
During the terms that I am teaching, 8 meals/week will be covered. This will leave 13 meals/week that I will need to purchase at about $2 meal. That's $26/week for food for the weeks that I teach x 4 weeks/month = $104/month.
Transportation: $100/mo
Taxi's, tuk-tuks, buses and the BTS (Bangkok Transit System) will take a chunk of Baht from my monthly budget. It is polite and culturally appropriate for the "farang" (white westerner) to foot the cab bill if travelling with students. This is a way for me to be generous and to build relationships with the people I spend time with there.
Ministry Costs: $100/mo
It is customary to take students out for coffee/tea and treat them. Also, many teachers, at the end of the term, buy gifts for their students (and vice versa). There are also breaks between terms when teachers are invited to visit their students' families-- this comes with the cost of hostess gifts and travel. All of these ministry-investment costs come from this part of my budget.
Phone/Internet: $30/mo
This will help me stay connected with the world... and in case I get lost in Bangkok.
Classroom Supplies: $20/mo
Paper, markers, posters, pens and pencils, etc. 
Medical/Emergency Evacuation: $30/mo
In case there is a natural disaster or I suffer traumatic bodily injury... this will help get me out of the country or flown somewhere to be treated. Also, it will pay for any doctor's visits I may need to make while there (but let's pray I don't need to do that!).
Necessities: $50/mo
Toothpaste, shampoo, toilet paper, laundry detergent... you get the idea!
Misc.: $20/mo
I don't know what all will come up while I am there-- but this is my "flexible" money. In the time between terms when I am not receiving meal vouchers from the BSC, this will help cover my food. Or, international banking fees. Or, any other category I've forgotten!