Between which locations are transfers using Bitcoins cheaper than PayPal for 100$?
In the euro zone MtGox is not cheaper for small payments like $100. Depositing and withdrawing euros both include a fee of ~1.20 euro (10 PLN) + 0.60% for the transfer.

I don't know what deposit/withdrawal fees are for US dollars, neither if there are exchanges that do not require this fee. I know Intersango does as well.