The point values for the various letters are as follows:
Note that if a letter tile has a multiplier bonus (e.g., '2X' or '3X') then the value of that letter will be 1 rather than the value listed in the table. For example, you can see in the table that the letter P is usually worth 3 points and the letters I and E are usually worth 1 point each; thus the word "PIE" is usually worth 3+1+1 = 5 points. However, if the P is on a 3X tile then the P is only worth one point and the word "PIE" is worth 3 x (1+1+1) = 9 points.
Note also that if a letter tile has +10 bonus, then the value of the letter is not increased by 10; it simply is 10. For example, the letter P is usually worth 3 points. However, if the P is on a +10 tile then the P is worth 10 points (not 3+10 = 13 points as you might possibly expect).