With some good shopping you can find a CZ75B for $375, and a kadet kit for $250.
Cheaper if you're patient.
A little over $600 gives you some good shooting.
If you're squeamish about filing on your new kadet adaptor, factor in the cost of fitting the slide, prolly about $40.
Another reason to get the Kadet adaptor, not the whole gun.
The 75b comes with fixed sights.
It will almost always shoot to point of aim.
The kadet adaptor comes with adjustables.
If you get a the kadet gun, and a 9mm upper, you might have to upgrade the sights on the 9mm.
Keep in mind that you could buy a good .22 for less than $300, but not one that feels EXACTLY like your 9mm.
If a local shop would give you a package deal, that would be cool, too.
A word of warning.
If you're gonna use the 9mm for a kadet, don't have a trigger job done on the nine, or you might have light strikes, and may have to buy a new sprinig.