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).