$6/lb is not a bad price at all. My LFS charges nearly twice that. Once you factor in shipping, you will probably end up paying the same price if you buy it online, unless you get it on sale somewhere.
$6/lb is pretty good! I bought my rock online and I wish I would have gone through a local store somewhere. Shipping can be really expensive online because at minimum you want it with 2 days if not overnighted to prevent die off. My rock came wrapped in wet newspaper in a box which helped a little. Another advantage to buying at a LFS is if the LR already has good coralline growth that is great. If you order online it seems people are lucky to get the rock with the coralline. One way to go is to buy good base rock/rubble online and add some high quality LR from your LFS. I had a friend do that and he bought one nice sized piece coated in coralline and his tank exploded with coralline algae! My tank has been up for about 1.5 years and I have some coralline. his tank has been up for about 4 months and he has 10 times as much as I do! One thing I have learned with saltwater is that cheaper is nice, but not always better. Especially in the beginning.
I found mine online, but it was 4 hrs away, so I ended up hopping in the car and going to get it myself, that way I could pick out the pieces. The guy could not believe I would drive 4 hrs to get rock, so he discounted me a little more, so I ended up getting 50lbs for 300 cash. Maybe look for someone online, who is set up relatively close to you, you might be able to do the same thing.