I usually buy from my LBS. As far as cost to ship, well that entirely depends on the dealer you buy from. Most flat-rate places won't charge a flat rate because the cost exceeds the contract with the shipper. Another factor is the distance from the shipper. I think it would run anywhere from 20-30 dollars based on what I have read.
I was looking at a few different sources, 50 Pound Sack had their shipping listed as free, BMW had theirs listed for $6.99, NB listed their for $36.76 (holy s***), so it can vary greatly depending on where you buy it.