Seems like Vung Tau in Vietnam is the cheapest place at the moment for a refresher at least. Thailand will never be the cheapest even if it charges the same as other centres as you pay 7% VAT on top. Thailand was about Baht 32,000 for a refresher, Vung Tau about USD$500 , a big difference. Spend the USD$200 on a ticket from Thailand and still have USD$300 to spend in VT.
As always, time and convenience, available dates, personal preference etc,etc, play into the game