Pricewise, online will be your best bet. However, it might be beneficial to check with your LFS. One, establishing a good relationship with them can help you out in the future, especially if say you had an emergency while out of town, they will be more willing to help someone who has good relations going with them. Also, the LFS where I just bought the rest of my LR at only charged me for up to 20 lbs. They said most people don't buy that much at once, and even let me just pick a few rocks out free of charge that I liked. I ended up leaving with approx. 35 lbs of LR, and only paid for 20. This might not be the case everywhere, but it's worth a shot. They also might give a discount for "bulk" buys, i.e. $4/lb for 20 lbs, or $3.50/lb for more than 20 lbs, etc.
If all else fails, you could buy part of your rock online and part from a LFS. One more bonus is that you get to handpick from the LFS, since you know best what will look good in your tank and what will fill that one spot that you just aren't quite happy with yet!
