My first job in a print shop, the owner gave me his pricing calculator. I've used it ever since. I should note, this is for cut decals from Oracal 631 and 651. Width of the vinyl roll + length of the cut (in inches) divided by 2. That's my base price. on a 24" roll, I keep my designs within 22", so say the design fits into a cut area is 22" x 50". I add 2" to the width and 2" to the length (for the rollers and extra length where it's cut). So that's 24+52=76/2=$38. This is already a decent profit. For me, I've gotten really fast at weeding my own designs, so I'll just round up a few bucks if it's a design that takes more than a few minutes to weed. For custom stuff, I'll round up a little more. Custom designs are charged at my $60 an hour freelance rate. Re-creating artwork in vector for me to cut is charged at $30 an hour rate.
Using this pricing calculator on my 24" rolls of 631, every 5 yards of vinyl sold pays for a 50 yard roll. 45 yards of pure profit. Also gives you some wiggle room to offer decent discounts when people make large orders.