What is your time worth? I think you should go a head and charge fair price. $75 should be for something that will take you a lot less time, maybe something like wash, clay, quick wax and tire dressing?
When you start to do work for cheap, people will be expecting that all the time, I'm a contractor, and I never give my work for less than what it should be, and if people don't want to pay the price, just let them go a head fine someone else.
I'm guessing that you want to do this to get customers and practice, on set of giving your work for cheap, I would just do a few of my friends and/or family members and take lots of pictures of the process, show them what you can do.
How do you thing people will react when the second time they come to you and you ask them for $250-$300 for something you did for $75 before? If you want to give them a discount, give it to them on the second, or third visit, (If they come back with in less than 30 days), that way the worse is already done (the correction).
Just my 2 cents...