In commercial terms, 2.3m is not a lot of money. Neither is 35 million for that matter. In a new purpose built building, the running costs for fuel and light will also be much lower, as will the maintenance.
The old Glan-y-Don Hall was not purpose built and parts of it predate 1878, when it was rented by Cheadle Royal Hospital. I am not sure if the information is easily available, but I would expect that very large renovation and repair bills were looming for many of the older buildings that CBC were using for office space. By moving people from older buildings (plural) into a single new building, will not only save money, but the CBC will also be able to sell off some of the surplus real estate, much of which had been old maintenance heavy, converted, Victorian residential properties around the borough. Some of these older building were extremely dilapidated.
When I read things like 'Shock Horror Council Spending' in local rags like the North Wales Weekly News, or Conwy Daily Post, I expect not to be accurately informed.