There is always a range of prices.
Take these prices with a grain of salty ink.
They are based on a range of what other people charge.
For the S-XL, between 4.85 and 5.60
XXL 6.50 to 7.40
XXXL 6.60 to 7.55
XXXXL 6.75 to 7.70
Depending on who the client is, how stiff the competition, how much you want the job, but make sure you will make money on the job.
For that many shirts, production time will be important to note.
How many shirts can you really do in an hour, hour after hour. that's not 800 shirts, it's really 1600 shirts (both sides) How much an hour should your shop make?
How long will it take someone to unpack, count, layout, re-layout after printing, and rebox. How long and how much an hour.
You can take the cost of the shirts X a reasonable markup, 15% to 20% markup
Plus the hours to print X hourly rate
Plus the hours to layout X hourly rate
Plus overhead
Plus commission (paid to the salesperson OR yourself)
Plus ink and screen costs.
I imagine your goal is to make money, so don't give the job away.
Good luck and keep printing.
Steve