Re cost - you can get this stuff at Bulk Reef Supply for substantially less than a SeaChem product, but it is more than HD products. When I factored in the time of not having to search the world for the right product at the right HD, and worry, the BRS purchase gets better and better.
As far as alk, I don't see why someone would buy anything other than baking soda. It is food grade and really cheap. A bag that has already lasted me 2 years and probably has another 2 years left, cost about 10 bucks at BJs.
Re alternatives - while HD sells products, so do pool companies and spa companies. I have purchased the calcium supplement part from a spa source, and it looked clean. All spec sheets from the manufacturer seemed right on too. The cost wasn't bad either - a little more than HD price, but I could buy a 10 lbs plastic container instead of a 50lb bag.
David