The only relevant thing in pricing is what is it worth to your advertiser/ customer. Your costs are 100% irrelevant. You need to look at what the competition is charging (ie flyposters, door drop leaflets, local paper, radio etc) and how effective they are at converting to real sales compared to you. All mr advertiser cares about is "if i spend $xxx on these screens, will it bring me $yyyy in new profits".
You need to work up a business case on how many people will see the screens (and be able to back it up) and also some stats on how moving dynamic digital images are far more persuasive when well designed than 'lumps of paper'. Have a look at amscreen (UK based big outfit) - their site has some good case studies and stats aimed specifically at small local advertisers.
