Its a how long is a piece of string question I'm afraid.
Dependant on the weight of the lamb in december, which will be dependant on when you wean, how good your grazing is, worming program etc etc. It sounds like your trying to pre-sell the lamb before its been sent for slaughter, which IMO is quite risky. You want your customers to be happy and buy from you again, so you really don't want to over or undercharge.