Here is how I added an extension to a bicycle so I could haul boxes of vegetables from a farmer's market. The Xtends unit uses a discarded bicycle frame and some electrical tubing. My cost was under $20 but the Xtends unit could be built for free if you have a good recycle pile.
The Xtends unit can be removed to return the bicycle to its normal configuration.
The Xtends unit involves welding. For a similar conversion that does not require welding see: http://www.instructables.com/id/Homemade-Sport-Utility-Bike-SUB/. Step 9 has some suggestions on how to make the Xtends unit without welding.
NOTE: I have added Step 11 to show you a way to make the Xtends unit that is simpler and stronger. Please check it out.
This project assumes you have some experience disassembling and reassembling bike parts. See this Instructable (http://www.instructables.com/id/How-to-Build-Up-a-Bike/) to get some idea how this works if you are not already familiar with it.
This project involves welding EMT conduit. Please see my Instructable for suggestions on ways to do this with a minimum of risk: http://www.instructables.com/id/Welding-EMT-Conduit/