I have found that a good deal on 2-part chems is significantly cheaper then a calcium reactor. In fact the operating costs of a calcium reactor alone cost more then the 2-part dosing (if you get a good deal on the chems).
6 or 7 years ago I got a reactor cause I was tired of paying $25 for 1 Kg of calcium. It was cheaper to have a reactor back then. It is also easier to 2-part dose, less fiddling, less stuff to go wrong, etc..... If you think that all the parts on your calcium reactor will be trouble free for years you are wrong, maybe 5 or 6 years, but eventually you will have problems with your pump, fittings will get brittle, and your CO2 valves will have problems eventually as well.
That being said.....................I use a calcium reactor, not for the cost, but for the fact that I don't have to add anything to my tank for well over a month.
If you do want things like calcium and magnesium for a good deal, I am pretty sure there is a good vendor around here who sells them.
Just a side note, and something I may do on my next tank:
-get one container of calcium mixed with water and one of baking soda mixed with water.
-use a multi-head dosing pump to dose into your tank
might be a simple cost effective way of eliminating maintenance.
