We have done many jobs like this. Here's how we do it and you will have to figure out how long it will take you.
1- Paint the main color, up into the the area where the strip will be. This includes any cutting-in at the corner, bottom of the walls, around doors, etc.
2- After the main color has dried, typically overnight, you need to make a measured line for the stripe. Measure down from the ceiling 18 inches and mark with a pencil. Do this every few feet or couple of feet depending on a few things.
3- Use blue chalk line to strike the line on the wall. Make sure to knock off excessive chalk and only use blue chalk.
4- Now run tape along the chalk line. You can use blue safe release tape or frog tape. I like the frog tape.
If you are working alone the chalk line idea will be difficult to do. In this case make pencil marks every couple of feet and use these to guide your tape.
5- Now roll on your color for the stripe after cutting in next to the ceiling. Dry-roll a bit next to the tape, don't get sloppy.
The amount of time this will take, amount of paint and tape or whatever you need and a little profit for your company is the price for this job. These things depend on how much you want and the prices you get from your suppliers.
Hope this helps.