I don't know about the cheapest, but I believe the best is BatteryJunction.com. You get free shipping on Surefire lights, 8 free batteries with the L1, and a 5% CPF discount when you enter the code CPF2007 at check out. Great people with A#1 customer service.
The L1 I like for the rugged feel. The knurling is like none other I have felt, that is except for my other Surefire's. It is like velcro that attaches itself to your hand. The engineering and machining that goes into this thing is just simply amazing. I love the simplicity of the switch with its two levels of either momentary or constant on. The size is perfect for either pocket, lanyard, or holster carry. It has good run time for a single cell light. The output is just amazing, and equals the output of my 120P. Although I don't like to abuse my lights, I feel this little gem could take a lot of abuse and just keep on trucking with no problem. And, if it does ever have a problem, Surefire backs it up with one of the best warranty's in the industry. I highly recommend this light....Not only as an EDC, but as a tool that you will be able to depend on for years to come.
Rob