Reducing price for quantity.

Hi, I need some opinions on this please.
I run a small shop with a one head Barudan machine. Always trying to find new and larger customers. I usually do a quantity of around 20 pieces per customer. But some luck came my way and I have a job for 500 pieces to do. Just the back with 13,700 stitches in the design. All text. I can do two and a half pieces in one hour. Thats 210 hours to do the job. (one head) .My normal price for 1000 stitches is between 75cents and $1 on supplied items.
My question is; how much should I lower my price for the 500+ pieces? Normally this customer uses a big embroidery firm. So I need to know what a "Big " embroidery shop would charge for this type of work. I dont want to loose this customer by over charging.
Id appreciate your thoughts please.

Thanks
Earl