So I’ll make this quick, “rule of thumb” conversion is below. It worked well during my stay in 2015 (spoiler: still holds up).
So all you do is:
Multiply the THAI BAHT Price by 3
2. Move Decimal point to the left, twice
300 THB = 9 USD
Because 300 * 3 = 900 == $9.00
Now let me see how this aged with google conversion,
as of September 2017, 300 Thai baht converts into $9.05 it appears the rule of thumb holds true!
Hope this helps.