We have no idea on the window specs, no idea on the installation and what they are including so really impossible to give you an answer. The removing existing? Siding type and patching? Interior finishes? Ground level or second floor? Area of the country?

" Expanding the opening down only by about 6" ... so no new header or cutting needed. Just new sill. "

Still gets reframed under the window and you can't just chainsaw it off and throw in a new sill.

This is ground level and there is removal but no disposal of new window.... yes sounds like just chainsaw and new sill... only 6" larger, so pretty easy ? Why is new framing needed under the window (assuing you are saying all drywall under window would have to come off)?

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DEMO

1)- REMOVE AND DISPOSE OF EXTERIOR TRIM AROUND (2) FRONT WINDOWS

2)- REMOVE AND DISPOSE OF INTERIOR CASING AROUND (2) FRONT WINDOWS

3)- REMOVE (2) EXISTING FRONT WINDOWS

4)- CUT DOWN FRAME SILL, DRYWALL AND SINDING DOWN TO NEW WINDOW SIZE

5)- FRAME IN NEW SILL FOR NEW WINDOWS

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INSTALLATION

6)- INSTALL (2) WINDOWS SUPPLIED BY

A)- WOOD INTERIOR AND JAMB EXTENSION WILL BE PREFINISHED

7)-FURNISH AND INSTALL MEMBRANE BARRIER AROUND EXTERIOR OF WINDOW

8)-SPRAY FOAM BETWEEN WINDOW JAMB AND FRAMING JAMB

9)-INSTALL NEW CUSTOM RANCH CASING AROUND INTERIOR OF WINDOW

10)- FURNISH AND INSTALL NEW TRIM AROUND EXTERIOR OF WINDOW

A)- NEW CASING WILL BE OAK

" 4)- CUT DOWN FRAME SILL, DRYWALL AND SINDING DOWN TO NEW WINDOW SIZE

5)- FRAME IN NEW SILL FOR NEW WINDOWS"

Doesn't sound like he is planning use the chainsaw method. Who is supplying the windows, you or the contractor? If you are it sounds expensive. If he is it sounds about right. But is still depends.

The windows are included in that 10-12k price.

Full frame install, prefinshed interiors, casing, etc. Pricing sounds about right for a reputable window and door company, these are very expensive windows.

You can always get another quote or two to be sure, but you may find prices even higher for this scope of work from companies with the best reputations. You can certainly find lower as well, but be careful not to get into the "chuck in a truck" tier. The vast majority of complaints in this industry are from folks that took the price that was too good to be true, not from those that paid a fair price (or even higher) and got a good finished result....

Long story short though, yes, this is in the normal range for this product and scope.