Before you start searching for the cheapest citizenship option, you should ask yourself: "What's my final goal? What are the benefits I want to gain from this?"
If you are just looking for a "safe haven", a place to retire and live peacefully, or a country where you want to work and study, the cheapest options are definitely citizenship by descent (through your parents and ancestors), as well as citizenship by naturalization.
However, if you are a six- and seven-figure entrepreneur and you want a second passport because of financial reasons, you know very well that "time is money".
In that case, citizenship by investment may be your best choice, as those processes in some countries are pretty straightforward, fast and can be cheap, too.