I always check on line for anything I want to buy. Sort the product and find the price. Then go to my LBS and they "Pricematch" whenever they can. When you take postage into account they can normally come somewhere near, and if they can't I normally still get a sensible deal.
Just to give an example- I wanted a set of wheels for the Tandem. From online at a wheel builder they were £280. Got one pair from them and they were great. Next year went to my LBS and the best price they quoted were £320. This was with a later, stronger rim though so not exactly the same. Went to check online price and for the same spec wheel it was £320. Bought from the LBS and the new wheels are better. The LBS used a better spoke and first service on them was free. (Just retrue and check). That was 4 years ago and my LBS will always give me a good deal- but then I probably spend about £1500 with them each year.

Other thing is that my LBS want me to come back. They tell me if I am about to buy rubbish and suggest something more suitable, or if a cheaper item is just as good. Then there is warranty- No problems whatsoever. Then there is the Experience thay have, and the knowledge, and the Skills and.... The list will will go on. On top of that- How many times have you had to ask a technical question of an online dealer and had to wait for the "Owner " to come back to get an answer- or been told a load of rubbish by a Lad just out of nappies. Example- Wanted a set of bearings for my Mavic hubs. Can't get them as they are Factory fitted. My LBS wanted to know the serial no of the wheels and they had them in stock. Took the wheels in to check they were the right ones and 5 minutes later they had replaced them --Oh and bring the back wheel in next week and we'll have the new spokes in to replace the damaged ones that I had not spotted.