There are direct bus services from Bangkok to So Phisai. The cost vary a little when you take a charter service instead of taking the national bus service.
The fees are around 600 baht (around 15 Euro - Jan. 2012) a person. The VIP busses travel at regular schedules and it takes around 10 hours.
In most cases a small meal and water or soy milk are included in the fees. The bus will normally make one stop and closer to it's destination it will stop more frequently for people to disembark.