Yes mahogany can be much more expensive. The rest it's hard to know because there's so many variables and we can't see what you have on the job site.
I'm assuming they're double hungs? If so it could be a pocket window (full window inside your existing frame) or sash kits which use the same frame and jamb liners are inserted and new sash are put in place. Based on the price it's probably a pocket window. Either way they should be fine as far as rain proof if they're installed properly.

Thank you Fenestration_Taylor for your feedback. And helping me with my "window terminology".

Existing windows are deep brown aluminum single-pane single-hungs. At least 23 years old since that's how long I've owned the condo.

Replacements are the same look and color on the exterior as they need to match the same look and color as the other windows (of other five condos) on the front of this building.

Replacements will be double-pane double hungs Marvin Ultimate clad.

I looked up difference between sash kits, pocket window, and inserts. The exterior frame and sill are fine. The interior trim and sill are fine. Thus will be a pocket window (full window inserted into existing frame).

Proper installation and proper maintenance appears to be a frequent comment in the forum posts. Thus will keep that in mind with the contractor I chose.

Hi NC1924, I couldn't respond to your email because you told Gardenweb not to expose your email address. So I'm responding here.

I ended up with a mix of Andersen 100, 200, and 400 series, plus one Marvin Ultimate window. The Andersen stuff is nice for what it is. It looks nice and operates almost flawlessly.

I haven't seen the Milgard clad in person, because the two sources I was considering (Bruce Bauer lumber in Mountain View and Direct Buy) didn't have any on display.

After seeing the one Marvin window I put in my kitchen, plus all the ones on display on Bauer, I would definitely say: get the Marvin if you can afford it. Their windows are extremely well made and look beautiful. I'm very, very happy with the big, cherry awning window in my kitchen.

I wish I had sprung for Marvin for the rest of the windows in my great room / kitchen and master bedroom. In hindsight I could have afforded it; my employer keeps throwing more money at me. They would have looked nice all in cherry, and my plans to stain the Andersen windows to match didn't work out. (More on that below.)

I know nothing about window economics, but I do do a bit of woodworking. Mahogany is very expensive wood and is also harder and tougher to work than pine or fir. A price delta of 2x doesn't surprise me at all.

Finally, go look at them in person somewhere, don't just use the internet and take people's word for things. With windows, the fit and finish really matters and you can't get a feel for that on the net.

One thing to check when you look at them is how much of the unit is wood. I ordered some Andersen 200 series windows and doors in stain-grade pine. The wood is nice, but at ordering time I didn't notice that the frame and tracks are plastic composite. The white frame would have contrasted too much with the cherry-stained wood I had planned, so I ended up painting the wooden parts white.

Laura