What will it cost me?
Blah - pay only for what you need.
"Ads" are usually sold in 8 second "slots" - 'digital' is a very 'fluid' medium so you can slowly feed out your message to get the best impact.
Decide how much you want to spend ($1/day or $1,000/month etc)
Decide what locations you want to spend your budget over (city or suburb)
Press "Go"
Get better results than Google Adwords from the immediate conversion benefits of directing your prospective customer into your business as they walk or drive by