Price Tag for hiring a monster as mercenary?

I’m designing a game where players could hire "any" monster as a missionary and fight for him/her.

The only restriction is probably the alignments. After all, a CE monster will NEVER work for a LG character! Or rather, a LG character will never "hire" a CE monster! (All he/she want is kill the xxxx out of him/her/it!). And vice versa.

That’s all good! The only thing I can’t figure out is the "price"!

Apparently it should be based on their CR, but I really can’t figure out the math.

I want the player to hire higher-CR monsters as much opportunity as possible, but I don’t want it to be "too expensive"!

Otherwise the players could just hire whole bunch of low-CR monsters and zerg through everything, what’s the point of hiring any expensive high-CR monsters!?

And if it’s too "cheap", any "challenge" I put forth for them will be "too easy and meaningless"!

I know I could exploit the Leadership feat to put a "cap" on it, but at higher Leadership, the amount of low-level followers are in high double-digits and are in very good fit for the definition "Zerging"!

So I’m really at the end of my rope here, and I can’t find any alternative rules online talking about this. (Most of the things I found is "xxx discount on Amazon"! ~>3<~)

Could somebody PLEASE be so kind and help me out and guide me to the right way!?

Much appreciated!