Basically "It Depends". (Shopping/Design Time = rate 1) + (Installation/Cleanup Time = rate 2) + (Cost of Materials * Markup). The size of the pots, and the ease of accessing them (rooftop? busy front door requiring after-hours installation?) affect installation and plant choices. Consider the cost of the plants and whether you can get them at wholesale nurseries or discounted retail at local garden centers. Consider your time in sourcing the right plants.

Is there someone who will water the pots daily and deadhead weekly, or will they look like crap after a month and meanwhile your sign is still attached to them? Upsell them a maintenance contract for the summer. Define when your agreement with them ends so you remove your sign. Good luck!