paintsmith,
thats a hard call being the stripes will be 8-10 ft. off the floor. Depends on how many of the verticals you are doing , does it cover all the walls, one wall, two walls. How long are the verticals going to be. How long in length is the horizontal stripe going to be, and on how many walls.
The biggest issues I see in pricing for the verticals is having to do it all on ladders/scaffolding and the amount of time it will take in the inital prep of measuring, taping/taking down, . The painting part will of course be the easiet (still not easy enough being it will be all up so high). Hope you have one of those good neck braces or a good chiropractor afterwards.
For the high verticals here are some guidelines from Paint Contractor's Handbook,; over 8' to 13' = 1.3 times the price; over 13' to 17' = 1.6 times the price; over 17' to 19' = 1.9 times the price; 20' - 21'= 2.2 times the price. The Handbook called it the "high time/difficulty factor.
As for pricing - quote a flat fee but figure out hours and material costs to you. Just try and make your best guess on the number of hours it will take you and x that by what you need to make per hour , then add materials. Work the decorative bid from several directions (how much time & materials? what does that add up to per sq.ft? Use time and sq.ft. to cross check yourself. Included in total, set up, prep, measuring , tape, applying tape, painting, any prep from assuring a clean crisp stripe line, working overhead, scaffolding, cleanup, etc. . Most of all, the taping will take the longest. glad they are 12"- .
Don't know if any of this helps or not.
Best of luck.