An organised Holiday will charge between £1200 and £2000 depending on accommodation required, and has a minimum of 10 people (so they are looking at bringing in £20k before payng out for hotels etc)
I'd throw the idea of a cost per mile "out there" with a transport cost to and from, or a flat day rate all in.
10 - 14 days I assume that would be either 5 days, rest, 4 days (10 day) or up to 4, rest, 4, rest, 4 (14 days) therefore between 110 miles a day or down to 80 a day.
I'd suggest that anthing be split to number of days, and location. Such that the longer the time, the lower the rate card. Two days from JOG would have to include transport to and from, plus the acutal ridden miles. In Central England could do just the miles ridden rate, and longer periods would defalt to a day rate.
Something like:
Days South North (of Manchester)
1 Per mile, plus transport
2 " "
3 Day rate Day plus transport
4 " "
5 " "
6 " "
7+ Day rate