I would suggest that in order to stop unscrupulous people doing the same as you unfortunately have had done to you that you see if your vet will consider early neutuering your kittens. This will prevent any bybs using your cats to breed willynilly from. You could include the cost of neutering in the price of the kittens. I usually pay around £45 for a female spay at 14 weeks - a little less for the boys. You hang on to the kits for at least another week -10 days after to ensure they are over their op, have a final health check at the vets and then they can go to their new homes. A lot of folk are happy that this has already been done and the recovery of the kittens is amazing.
Obviously this is something you will want to discuss with your vet in detail - the pros and cons but it is well worth giving it some thought.
You could perhaps price your kittens around the £175 inclusive of neutering.
