(There is a formula. I have a poor memory for formulas but am good at transforming English into them, so I just remember the English gloss.)

I'm decently impressed that Wolfram was able to spit out something equivalent to the right formula with only a little massaging of the inputs from me:

http://www.wolframalpha.com/input/?i=sum+of+n+from+n%3Da+to+...

  (loop for x from 1 to 365 summing x)  ==> 66795

Unless you want it for this: http://www.wolframalpha.com/input/?i=sum+of+n+from+1+to+n

  (reduce + (range 366))

  reduce (operator.add, range(366))

   sum(range(366))

  (sum (range 1 365))

  (defun range (start end)
    (loop for x from start to end collecting x))

  (defun sum (sequence)
    (reduce #'+ sequence))

Math/brain fail.

I'm sitting on two other niches for basically this business model - planning on moving overseas next year at some stage, though, so will wait until 2011 to implement (by which I mean October 2010 to start marketing).

I'm sure we could all come up with some variants.

Edit: It also helps that it's most attractive to retail / mass consumer products, and that these are most in demand in the holiday season, and that the holiday season coincides with the end of the calendar year, so the most expensive dates are also the most valuable.

Not bad for a year.

Edit: It is. I just ran the code again, and the payment for day 365 is 1.28331e+15