The cheapest source of good commercial
juice IMO (which isn't that extensive considering I started DIYing by week 2) is
Dark City Vapor . They have greater options than most vendors for strength and if you want you can write whatever you want in the purchase notes box and, if it can be done, Bill will do it for you and put a cherry on top. $2.50 for 5ml all american made juice.
If you want juice for cheap you need to learn how to make it yourself. It's as easy as mixing a drink, just a little more danger element to it if you are a klutz and spill your nicotine solution all over yourself. It's costing me about 20 cents a day to vape and it's getting cheaper all the time.
For more information on how to get started DIYing your juice read my blog, The Vaper's Handbook:
http://www.e-cigarette-forum.com/forum/blogs/shoganinja/2998-vapers-handbook.html