"It is cheaper to buy a pair of 2400mhz ram then a pair of 2133 MHz" I believe you mean "than".
Perhaps but pricing is always a bit of a guessing game.
In any case, you may not gain much by doing so.
And if the memory modules are not matched it may all be for naught.
Start by looking at the motherboard's manual and the allowed memory configurations with and without OC.
Then look at the memory modules - both installed and being considered.
There may be an acceptable installation/configuration that will work. As to improving performance - likely not noticeable.