It is up to your plan!
If you just for relax, $2000 (~THB9000) is enough for one person.
Message ranged from THB300-THB1000 per time.
Spa ranged from THB1000 - THB3000 per theropy. (2 to 3 hours)
Food in local food court is very cheap. Each time just cost less than THB100 per person.
Remember, when you leave Thailand, there may be as addition tax THB700 per person. (I've just experienced this at Phuket, Thailand in last month). You need to keep some of the money for this.
For money exchange, the rate is higher at local shop (1: 4.5X) rather than that of Bank (1:4.4X).
Wish it can help you.