"Bah" is an onomatopoeic (read: onomatopoeia) word that is generally used to portray a sense of frustration.
The word "bah" differs from other popular internet shorthand such as "lol" because it is not an accronym, but rather a word that sounds like it is spelled.
You mean it's going to cost me $200 to get my car fixed? Bah! That's a ripoff.