Windows are simply measured and estimated based upon the count, so to estimate your windows you simply need to count the # of windows you need to replace in the property and multiply the count by an average cost per window.
On a 1,500sf rehab project you will likely have between 10 to 15 windows that need to be replaced. Replacing each window typically takes between 1 to 2 hours, so a window crew can usually complete an entire home window installation in 1 to 2 days.
The most obvious factor that influences window pricing is the type of window material being installed. Vinyl windows are the cheapest ($100 to $300 material), whereas custom wood windows can be $300 to $1,000 each.
Fixed windows are cheaper than operable windows that open and close.
In construction, there is a term called economies-of-scale. The larger the project, the more efficient the contractor can be installing the windows. The unit price (price per window) to install just 1 window would be way higher than the price to install 10 windows. Ex: A project that has one window may cost $500 ($500/1 = $500 per window), whereas a project with 10 windows may cost $3,000 ($3,000/10 = $300 per window).
Standard window sizes (~2 to 3ft wide x 4 to 5ft tall) are cheaper than large windows (3ft+ wider). Custom window sizes will also be a premium to in-stock windows.
If you are changing the size of the window rough-opening this will require additional work for framing, siding, & painting.
Window contractors will generally provide an all inclusive bid to 'furnish and install' all labor and window materials on the project. Window procurement is a time-consuming process that involves field measuring all of the window rough openings and purchasing the correct sized windows. You do not want the risk or responsibility of measuring your own windows, so always rely on your Window Contractor to furnish and install all window materials on your rehab projects.
Here is an example of how to write a Scope of Work that insures your Window Contractor provides an all-inclusive bid.
All Window work (including window materials) are to be Contractor Purchased, Contractor Installed.