When a house has ac units in all the windows it’s a sign that house has no central air or it hasn’t been updated since the original install. To get a full hvac system installed and permitted with returns, supplies, furnace and a condenser is 15-20k. If the house has existing system and your upgrading it can be 6-10k depending on the hvac contractor. If your using these window units which work well for time being you have to make sure the condensation is not dripping inside the window and damagin