Best place to open custodial account for 1 time purchase of stock (small $) for minor?

My son, just turning 10, wants to use $30 of his birthday money to purchase stock in a certain company. It will have to be a custodial account, as he is a minor, will sit untouched at least 8 years (till he's 18 and gains control, I hope longer!). I offered to pay the brokerage fees-where is the cheapest place to do so? I don't want to spend $20 to help him purchase $30 worth of stock. It looks like sharebuilder does them for $4- if you go on 'automatic deposit', which we're not interested in right now. So...cheapest place for one time small $ stock purchase for a minor? TIA!