I've only recently become aware of this US Department of Energy program:
http://www.windowsvolumepurchase.org/ Note, that the minimum volume is 20 windows, and applies to low-E storms as well as U0.20 windows, but apparently you're allowed to mix sizes & types to come up with the count, but I think you have to stick with one vendor.
At least one of the vendors (Larson) who distributes through box-store chains offers the volume pricing for 6 or more windows on low-E storms if you click on the link for a printable voucher:
http://www.larsondoors.com/storm_windows/vpp Putting a low-E storm on a serviceable U0.4-U0.5-ish uncoated double pane that's still in good shape delivers ~U0.28-U0.30 performance for a fraction of the cost of a U0.30 replacement window, making it a fairly cost-effective upgrade compared to replacement-windows.
Putting a low-E on a tight U1.0 single pane (even a tightened up antique) delivers ~U0.33-U0.34 performance, compared to ~U0.5 with non-coated storm. The delta in price between a low-E storm and a brand-X uncoated version is small (especially if you buy under this program). The payback period for low-E storms vs. brand-X storms on utility savings is cut roughly in half due to the enhanced performance(!), despite a slightly higher cost. In most markets that payback is going to be about 5 years (simple) according to DOE numbers (vetted by Lawrence Berkeley Nat'l Labs and other in-situ testing) but in high fuel/utility cost situations it can be much faster. (Heating with propane, or oil, anyone?)
Lots of links to more in-depth info is available on this site:
http://www.low-estormwindows.com/