The kits are nice to start with, but if you know what you are doing, you can just read a recipe and buy what you need. Maybe tweak the recipe if you like. I am pretty sure that buying grain from the buckets at the LHBS is going to be cheaper than buying a prepackaged kit. But, some people just want to grab a box and brew beer.
The cost of brewing is roughly like:
Bulk Extract ingredients purchase
All Grain Kit
Bulk All Grain ingredients purchase
Group Buy Grain
From costs most to costs less. I cut my spending on ingredients in half by going AG and buying sacks of grain and hops by the pound (and growing some).
If your takin' your girlfriend out tonight
You better park the car well out of sight
'Cos if they catch you in the back seat trying to pick her locks
They're gonna send you back to mother in a cardboard box !