This is a good for everyone to think about. I tend to be on bannertime's side, each process has a minimum and design and layout should be a separate issue. Too many clients can draw out that part of the process.
When I worked in a shop we had a board with 30 or 40 fonts divided by styles along with a letter height and basic color chart. This took out the small order layouts for boat #s and real simple stuff. Using this process we were at $25 on cut vinyl for walk-in's. We helped sell this minimum by letting the customer know that on smaller fonts we would cut extra and they get whatever made it through production. This way when the guy weeding (lowest guy on the totem pole) lost a letter we didn't have to reload the vinyl saving time in the long run. Vinyl is cheap, billable labor isn't ($1 a minute).
With printed vinyl you can't do anything less than 1 hour of your hourly charge even for good customers. And, that's if the art is correct and ready to print. Think about each step you have to go through; talking with the customer, emails, files, rip, load material, print, dry, load the laminator, laminate, cut down, invoice, deal with the customer again, collect... Again we offer value for those minimums for our customers. They want qty (2) 3" decals we let them know we'll print as many as we can fit in 3" x 50" so they'll have extra. It doesn't seem as bad when they are paying 5 or 6 dollars each for 15 decals and we were going to throw that extra away anyway.
Each job is thought of as a separate job. Batching jobs together to save time and money is a juggling act where the real money is made or how you stay completive on the larger jobs. Those savings aren't typically passed on to walk-ins.
For now I am an install only shop and I install on site with a $125 minimum. I know shops that have a $250 minimum. They run guys in 2 man installation teams.
Before establishing minimums think about each part of the process and how long it takes. How many times have you done the task and wish you charged more, how small customers interrupt large important jobs, and how many of them are truly a pain in the azz.
Most of us have done a small job for the regular joe who turned out to be a successful business person and ended up getting larger contracts. Just because you have a minimum doesn't mean you have to be a jerk, or always enforce it. Use good judgment and when you do enforce it sell it.