Getting quotes for replacing all windows in a northern climate (MN) 30 year old 4 level split home with original wood double pane casement windows.  About 15 windows and 8 sliding patio doors on the east side of the home.  Seven of the patio doors are on a four season room which is now definitely not inhabitable all 4 seasons.  Typical 2x4 R-11 or R-13 wall construction.

Looking mainly at vinyl or fiberglass low E fixtures.  One manufacturer is offering a special free upgrade to kyrpton vs. their standard argon filled IG.  I believe their premium is typically $100-$150 per window w/krypton.

Is there really that great of an insulating value between the two gasses?

This is from www.energyguide.com

Gas Fill
The space between the two panes can also be filled with argon or, less often, krypton gas, which insulate better than air. Krypton is somewhat more effective in windows with less space between the panes (1/4 inch to 3/8 inch), so it is often used in windows with multiple air spaces (such as triple-pane windows) to keep the thickness down. Windows filled with air or argon work best when the space is about 1/2 inch. Windows with krypton are usually more expensive, both because krypton itself is expensive and because the designs tend to be upper scale. Argon is nearly as effective and does not add much to the cost of a double-pane window.

Regardless which gas you select for filling the windows, you should check with the window manufacturer about the type of spacer and sealant used to keep the gas in the window.  I have seen estimates for gas leakage from 1% to 8% per year.  The amount of leakage primarily depends upon the quality of the spacer and sealant.

It should be pretty easy to look at the specs for their gas filled vs. krypton filled glass. I suspect the performance different isn't huge in standard-thickness double-panes, but will be real enough on triple-pane goods. If it isn't enough to make significant difference, don't bother sticking with that manufacturer over another's better-featured units of similar or lower total U-values. Even the best insulated glazing can be defeated by the frame it's packaged with.

Argon is relatively cheap & plentiful- it constitutes about 1% of the air we all breathe, and is easily obtained in quantity by compressing, chilling the air to a liquid, then decompressing or warming the air in a controlled fashion in a "fractional distillation" process. The argon is easily separated with decent purity, since is boils off at a lower & distinct temperature from most of the other gases.

Krypton is about 10,000 times rarer- about 0.0001% of the atmosphere, but it's boiling point is higher than most of the gases, and comprises the bulk of the liquid left after the O2 & N2 have boiled off, but is still contaminated with xenon, which requires a different type of separation process obtain pure krypton. I suspect the krypton used in windows is the "dirty" or ~1% xenon goods. (The gas used krypton & krypton-argon "white-light" lasers is of-necessity purer.) But it's all pretty pricey- if it isn't buying you relevant performance, there's no point to using it.

One thing you can do is this:
If you know the manufacturers model number and the glass make ups, go to www.nfrc.org and look up the glass code(CPD #). This will show you the U value and the SHGC.
The U value is the inverse of the R value(1/U).
This should help you in your decision making process.