Hey DeeDee, that sounds like a bug. You definitely shouldn't be getting charged more than the advertised price. Could you please email email@example.com? I'm a developer, so I can't do much about support tickets, but they should be able to help you out. I'm sorry that happened to you!
When I got the message about the barracks and went to the market, the price indicate was 240 gems. When I purchased it, the price subtracted jumped to 300. That is not right. Classic bait and switch. 60 gems is a big difference.