If you want to be serious about shopping and bargaining then it is important to learn Thai numbers. In department stores all prices are marked but in outdoor markets you will have to ask for the price and then try and make it cheaper.     
One
nueng Two
song Three
sam Four
see Five
har

--------------------------------------------------------------------------------
    
Six
hok Seven
jed Eight
bad Nine
gao Ten
sib

--------------------------------------------------------------------------------
  
Eleven
sib et Twelve
sib song Thirteen
sib sam

Once you can count 1-11 the rest will be very easy. In fact easier than English. Thai students are very good in learning Math. This is partly due to the way the numbers are named. Take thirty as an example. To us it is just a name but to a Thai student it is three blocks of ten. In Thai thirty is "sam sib" or literally "three tens". Thirty six is "three tens six".

From twelve to nineteen you just repeat the same formula:

14 = 10 + 4 (sib see)
15 = 10 + 5 (sib har)

   
Twenty
yee sib Twenty One
yee sib et Twenty Two
yee sib song Twenty Three
yee sib sam

Again, from twenty two to twenty nine you repeat the same formula:

24 = 20 + 4 (yee sib see)
25 = 20 + 5 (yee sib har)

Thirty
sam sib Forty
see sib

By now you should be able to count all the way to one hundred.

60 = 6 x 10 (hok sib)
70 = 7 x 10 (jed sib)

One Hundred
nueng roi Two Hundred
song roi

--------------------------------------------------------------------------------
 
One Thousand
nueng pun Two Thousand
song pun

--------------------------------------------------------------------------------
 
Ten Thousand
nueng muen

NOTES:

All shops use Arabic numerals so it will be easy for you to read. The only time you will see Thai numerals is in a school or when there is a two price system at a tourist attraction.
If you haven''t learned all of the numbers by the time you arrive in Thailand don''t worry. Many shopkeepers will show you the price on a calculator!