Wow those are really high prices. How big a unit are we talking here? Around here, they are typically under $2,000.00 Cdn, installed for most brands. I'm not familiar with Armstrong, but I know a few AC mechanics, and they all shy away from namebrands such as carrier, lennox, and Trane. The main reason they don't like them is their use of proprietary parts, which typically cost several times the price, for the exact same part from a supplier, but without the Brand part number. I hesitate to use the name generic for these parts (over the counter might be better) , as they are often the identical part, manufactured by the same company. The difference to the repairman is availability, and lower cost. So, it is possible that the installer is honest and believes what he is saying. I just can't believe the price.