50% is about right, I wouldn't expect any more. Ram lamb will have a lower KO weight if he is entire.
Few points here- if your lambs are overfat then charging per kilo may not be as fair as it sounds, people don't want to pay for fat. We too rear around 15 tame lambs per year and kill all of them by November, worked out it costs roughly £80 to rear, kill and cut a lamb- they are heptavac, wormed, flystriked etc. 20kg bag of milk powder is £36, we use 1 bag to 2 lambs, wean at 6 weeks, on a Shepherdess feeder. Our cost per lamb this year has been a little more as we had one put down post weaning due to white muscle disease, lost one to vit B deficiency and had to have the vet surgically castrate a ram lamb in June (long story!). We also kept 1 well grown ewe lamb who is now running with a ram. Our abattoir put up kill from £13 to £18 thanks to skin prices being too low, so now for kill and cut we pay £30. We bought 8 shearling ewes in August so this year was our last year rearing tame lambs, after 5 years I am fed up of mixing endless amounts of milk powder !
Have you weighed your lambs? They can be very deceptive, what looks like a heavy big lamb actually can be the opposite and vice versa. I would say for your first year sell per 1/2 lamb and do a flat fee per half rather than faffing around with weights- then if they are a little too fat people aren't paying for fat. £9/kilo for breast, shoulder is pricey, I think £80/half for a 10-12kg half lamb is about right- I know we couldn't get much more.