Here are my thoughts:
You're looking at $300-$500 for a day's lift rental.
Do you have safety harnesses or will you be buying/renting those?
How far away is it (gas/travel expenses)?
What do you have that will haul a 5x5'
sign to the location? (truck/boxtruck?)
Do you need blankets, etc to be sure the faces aren't damaged during transport?
Do you need lifting slings/clamps to pick up a 5'x5' face from the ground?
Will you need an assistant/spotter on location? (once you're up in the lift with the panels, it's a PITA to get back down for something you left in the truck.
Are you replacing screws/hardware in the panel? Cleaning the inside of the panel or
sign panel trackways? (these can be full of crud and rust)
Are you replacing all of the bulbs in the panel & the exposed bulb sockets, since the owner has removed the panels and exposed those elements to the weather?
Do you need to hire an electrician to service the electrical elements? (They are around $60/hr)
Who is pulling the permit/supplying drawings?
ALWAYS verify that all bulbs are working before installing the new panels, saves the customer a good amount of money to have it all done at one time when you're there & their new
sign lights up correctly.
As far as material thickness, you need to use the correct thickness for the tracks of the existing cabinet. This may require you to measure the tracks if the
sign faces they brought you are possibly not the original faces. If the material you are using is not the correct thickness you could have problems. Too thin = could blow out, too thick = will not fit into the tracks.
You are too low. At 1200 your costs are around/above your price listed.