Cheap & Effective Way to Soundproof Windows
There are a lot of options you could consider depending on your budget and opinion on "cheap".  There are acoustic dampening windows you could look into if you have a higher budget and don't mind replacing your window.  If you're sure that the main source of noise penetration is through the window, which is to be expected, then this might be the best / most permanent route.  The AudioSeal looks pretty promising, and there are some more cheaper options.  A simple search for 'Transparent Vinyl Barrier' gives you a lot of relevant results.  The quality and the transparency of the product will depend on your budget.  Curtains, and specifically acoustic cancelling curtains might be your best bet, but they can be pricey (and very cheap, technically you could use your bedsheets as curtains, although they wouldn't help much). 

Here are some relevant links you might want to check out:

http://homeguides.sfgate.com/block-out-noise-coming-through-window-31141.html

and

http://removeandreplace.com/2014/12/02/how-to-soundproof-a-window-and-block-noise/

Good luck!