What do you mean by cheap? Cheap to buy the chemicals, or cheap to get the setup going?
Dripping saturated Kalk is cheap, but very labor intensive - you can do this with Mrs Wages, a gallon milk jug, some airline and a flow control pinch. Manually dosing 2 part is cheap, but very labor intensive - you need two milk jugs, some airline and a few control pinches. If you automate either, it can get quite expensive if you have actual, useful redundancy to prevent a crash and quality equipment - you can spend hundreds on this.
If you factor in the long term CaRx is probably the cheapest... especially if you get a nice used setup for pennies on the dollar.
If you do decide on two-part, Dowflake and Baking Soda are your friend, like what RHF recommends - don't waste your money on stuff from BRS or the like which marks it up 10x, or more.