i thought that a charge on the house was an amount to be paid after the mortgag is paid off and before anyone profits. if there is no profit, then the person who placed the charge gets no money. I dont think you can specify when the house is sold, so if he needs to sell it now, and to do that he needs to drop the price, i don't think you get any say in that. there are lots of things that affect a house price, and even if smilar sized houses are on the market for more, it dosent mean thats what they will sell for or thats what his house it worth. Perhaps he is the only one being realistic in price - we have been looking for a house lately and nearly all of them have been on at a higher price then reduced (partly because people just can't seem to accept their house value has dropped)
I have no idea where you live, but find it amazing that 10k is going to get you 15% of one house price, let alone "buying them a house each"!
also how are you buying them houses if you are renting them out to other people? thats just building a property empire (or could be seen like that) and it's not all that surprising your ex dosent want to fund that for you (if indeed thats what he is doing)