Hi,
You might consider a more or less standard R3, double pane, low-e window coupled with a well designed thermal shade.
We did this one:
Bottom-Up Insulating Shades for Light and Insulation -- Our R8.3 Window
The combination of the multiple layers plus the edge tracks and a good fit along the bottom makes for (I think) about a doubling of the R value up to R6 ish. About $150 for a 36 by 54 ish window -- but since you would probably have some kind of shades anyway, its only the increment over a "regular" shade that should be charged to the energy saving.
We liked the results so much on this one window that we did all the windows along the front of the house and our big kitchen window with the same kind of bottom up/top down shades with energy tracks.
We like the bottom-up/top-down in that it lets up look out the top half of the window while still insulating the bottom half.
Gary