A single pane window has an R value of less than one. (.8) By placing bubble wrap in the window, your window?s R value will be around two. Keep it Solar has a detailed article on how much money bubble wrap will save you.
From Keep it Solar:
For an 7000 deg-day climate (northern US), and single glazed windows, the bubble wrap increases the R value from about R1 to about R2. This cuts the heat loss from the window in half.
Heat losses with and without bubble wrap for 1 sqft of window are:
Heat loss w/o wrap = (7000 deg-day)(1 ft^2) (24 hr/day) / (1 ft^2-F/BTU) = 168K BTU per season
Heat loss with wrap = (7000 deg-day)(1 ft^2) (24 hr/day) / (2 ft^2-F/BTU) = 88K BTU per season
The Price of Bubble Wrap
You can buy your own bubble wrap. It's inexpensive, but you'll save money if you salvage it. Furniture stores will have extra bubble wrap. Any large department stores will probably be swimming in the stuff. Call around to your local merchant friends. See if they can help. Bubble wrap insulation can last up to seven years.