B.M.
Hi S.,
It doesn't actually matter how much of a retainer he charges (well, except it matters to you!!!! lol). He will deduct the amount of work that he does from the retainer - leaving a 'balance' for him to 'spend'. As you drain down the retainer, you will get to his 'threshold' and he will ask you to 'replenish' before he does any more work for you.
Some lawyers use the retainer to 'get started' and others try to determine how much it will cost and then have the retainer cover most of the charges. That way they don't nickel and dime you.
It's the per hour charge you want to consider. Find out HOW he bills.....if he bills by the hour or by the QUARTER HOUR - this is actually VERY important. Some lawyers charge by the hour. So if he does 30 minutes or 15 minutes or 45 minutes of work for you at a a time it will cost you $150. Try to find someone who bills by the quarter hour.
You'll pay the same amount in the end, regardless of how much you give him up front as a retainer anyway. If you like him and you feel like he will do a good job that is what you should consider.
Good Luck.
B.