Please save yourself the aggravation and do not do window replacements from someplace like Home Depot. A very good friend used Home Depot...it was a nightmare as soon as a couple of the windows were delivered damaged.
We replaced 21 windows in our home 5 years ago in late winter. Companies are begging for winter work especially if you live in a cold climate so you may get a better deal by installing over the winter.
In the end we paid just over $13,000. Again, that was 5 years ago.
The energy costs have been awesome, but we had to replace our windows. More than one was rotting out of the frames...moisture between the panes...it wasn't good.
Get estimates, and ask neighbors if you can. We asked a neighbor across the street that did their windows 6 months before we did and I used Angie's List to pick the companies for the 3 quotes we got.