Replacement Windows and Sliding Glass Doors

[Jackson Sandland]

I have a rental property on Martha's Vineyard and I've been getting estimates for replacing windows and sliding glass doors. The property has one section that was built in 1921, though I don't believe the windows are original, and a large addition that was put on in the 90's. I've done one estimate where a contractor thinks we need to replace 12 windows and 16 sliding glass doors. The quote from a general contractor looks way too high at about $64k.

The replacement windows don't need to be fancy at all. They just need to be able to keep the heat in and operate as expected. Same with the sliding glass doors. I called Home Depot and the cheapest sliding glass door I found with high quality / high quantity reviews was about $500. I was quoted at $600 / door for installation. Total cost back of the envelope is $17,600. I saw reviews online of contractors taking 2 hours to install a replacement door but Home Depot said it can take half a day / door so thinking there is room to make that cheaper.

I've also researched window costs and found posts on bigger pockets where replacement windows were sometimes about $200. So $2400 for windows and $1200 for installation would be $3400. A far cry from $32k for those windows.

With that background, do any of these numbers make sense?

Can you buy windows / sliding glass doors wholesale / cheaper than sticker price for a rental if you are the general contractor / otherwise?

What is the most cost effective way to replace windows and sliding glass doors for a rental property?