I like Yankes answer. Because then others can have/use/tweak it. and the player can turn it on and off as desired easily without doing a bunch of nitty gritty file edits each time they just want it on or off.
But i agree the cost going up to higher a soldier would be fine. or, you know, lower the cost, but increase the monthly ask.
I 'higher' like 600 guys a month and immediately or not, fire 565 of them. then put the remainder through psi school, and see if i should cull more. its kinda silly!
maybe just pay these guys at the -end- of the month instead of for the right to look at their resume. X.X
also, as a mod, you'd at that point -know- you were altering your play through with it even if you kinda forget and come back to your save some 2 years later and are not sure why the heck you never get any low-end-soldiers.
But i don't know how to mod for xcom right now, so for the moment I'll use Delian's answer and keep a long ear out for if this does become a mod.