Regarding the actual question, discounting only makes financial sense when there are economies of scale in a certain business, meaning that the producer realizes some savings too by making things in bulk. You might have small savings in making a lot of dough or icing, but only if you have very large mixers that allow you to do large batches. I rather doubt you can speed up the most labor-intensive (and therefore costly) part, which is the decorating. You may, in fact, have added decorating costs if you need to hire people to help you. One might justify discounting the order, just to get his/her foot in the door, especially if s/he felt this would be a large recurring order. But I personally wouldn't do this unless I knew I had priced the order profitably to begin with and that I could handle the volume and sustain that pricing over time, as once you've quoted a lower price to a customer, there is typically no going back.
That all said, the best way to price any order, be it small or very big like this, is to go back to the fundamentals of figuring out how much it's going to cost you to produce the order; then price to ensure you cover those costs, pay yourself and any employees a decent wage, AND also leave profit to go in the bank or to fuel investments in your business. The "Business of Cookies" section of our blog has some excellent articles about how to do cost-based pricing, and pricing in general, so you might want to check it out.
Let us know what you decide to do and how it goes. Best of luck!