I like to support my LBS, and I do so when I can. But there are several times when that just isn't practical. I don't think my LBS stocks trainer mats, and I know they don't sell items like repair stands. They just don't have the market for those things. And it's not always just "a little more." My most recent purchase was for a set of Shimano PD-M647 pedals (and some tools that my LBS doesn't sell). My LBS charges MSRP, with the exception of previous year model closeouts. I got the pedals for $64 after discounts and pricematch from Performance (MSRP is $115).
If the difference is 5%, I'll gladly purchase through the store. And if its something I can "try out" at the store, or at least get a general since of appropriate size for (like shoes, gloves, apparel, et cetera), I'll almost always purchase at my LBS. But about a third of my bicycling-related expenditures are online. I think they both have their places.