Hi-
As far as a price guide.. I kinda wing it. Sorta.
1st- figure out your cost whether its per inch or per sq foot but always charge the full width of the vinyl. example. if its.. one 12"x5" decal.. charge for 24" x 5" - other wise.. when you throw that vinyl into the garbage.. you might as well be throwing money into the garbage.
2nd- know your customers. I do a lot of racecar vinyl, and I know who my competitors are.. I price around what they do... but then I have my secret card which gets me my business =D But, I usually use about.. say, for 1 color.. I usually 24"x 110" (for the 2 sides of the door and the roof) which is 9 ft worth of vinyl. I'll charge about $60 for it assuming its normal vinyl.. But if someone wants a bunch of decals out (lots more weeding) with the same amount.. im charging them more.. its way more work for me.
You really need to consider your cost of your material, whether its high performance or not, and how much time you'll spend cutting, weeding, screwing up, recutting, weeding very very carefully this time around (i had my blade offset wrong (with no idea of this) and spent many hours cussing out my machine and weeding) and then masking.. and possibly layering.
If i have to layer it, the price goes up. if its a small job where I can do it myself it wont be too much more, or if its a very large thing I have to layer, I have to wait for my boyfriend to get home to help me.. (i'm short, and cant layer large items such as 30" x 36" things by myself..

) has anyone found a trick to do this?) so then the price goes even more so up. I'm having to bug him, and usually then have to work later than i'd like to, plus its taking two people.
I know some people on another forum get $7+ per sq foot for a banner some even get $13 per sq. ft. It all depends on your area.. Where i'm at.. people dont want to pay more than $4 or $5 per sq foot.. And i'm not willing to work for free.. hence why i'm switching to t-shirts.
Dont settle for too little, dont work for free... and dont bother under cutting the other guy.. cuz then you'll lose business when you have to up your pricing cuz working 12 hrs every day wont pay the bills.. and then you're really screwed.
I'd say fair and competitive pricing is the best way to go.. plus, the other sign shops in the town wont hate you.
just my 0.02
Good luck!