If it were only to vocal processor that would be an easy choose; but as we know from this comparison https://www.bonnersmusic.co.uk/yamaha-psr-sx700-vs-psr-sx900.irs aside from their design these keyboards have little in common.

At least SX700 is significantly cheaper than SX900. What Casio does with its CT-X700 and CT-X800 and with Casiotone series clearly shows that it has nothing to do with the real cost of units but is just about giving a customer an illusion of a choice in which buying a cheaper one would be rather wasteful.

It’s true that money don’t grow on trees, but sometimes people buy cars saving money on motorized windows and mirrors or A/C, which is comparing to a total car’s cost is insignificant, but at least it gives them a feeling that they saved some money on extras.