Regarding your comment about charging wealthy clients more I have a couple of questions:

• Have you considered the risk factor of working a return with taxable income of $1M vs $20K? 
• What are the potential penalties in the unlikely event you make a mistake?  We ARE human and mistakes happen. 
• Do you give discounts to clients that don't make a lot of money to help them out?  If so, why would you charge them less than wealthy clients who can afford your fees? 
• Why are tax rates for successful people higher than those with low income?  Since their fees should be the same shouldn't they pay the same tax rates?

Usually, my fees are higher for wealthy clients more because their returns are more complex, but the risk factor IS a consideration, as well.   My billing rate is $250/hour for any time above a minimum fee I set for returns with business income/expenses (Sch C, Sch E and 2106) vs returns without.  I will usually charge $125 or $250 to amend a return, based on the complexity. 

For the person that said it only takes 5-10 minutes are you also factoring in the phone calls, e-mails, tracking the amended e-file, notifying the client when has been accepted, are you printing a paper copy or providing a digital copy?  I doubt anything but an e-mail every takes only 5-10 minutes.  I'm terrible at estimating the time an engagement will take, and it always seems to take longer than I expect so I pad the time.  You can ALWAYS charge less and look like a hero, but it's hard to go back and ask for more.  PLUS, if you give a range of fees the client will only hear the lowest number.