Any mic will do the job. I use an EV dynamic mic that I found 2nd hand in a music store and paid next to nothing for it. It´s hidden under the console so I don´t have to see it. Buy whatever mic you find cheaply second hand or new and the only limitation is how ugly it can look. And don´t get a condenser unless you´re able to feed 48V to it. The quality of the mic for this purpose is not important. Or buy a Samson studiocontroller 2nd hand and use only the talkback mic from it ! Takes up less space than a micstand. Or get a AKG miniature mic. Or a cool 50´s radio-speaker-mic with original tabletop stand :)

my friend uses a baby monitoring system for his talk back. No joke... it works great!

I'm not sure why it really matters what you're going to use for talkback, but... If you want an archetypal TB mic, check out the AKG D55.

Nady Star Power 1. Like $7.99 or something. Has on/off switch. I keep it on a stand next to me desk. It isn't even sensitive enough to make the monitors feedback if you accidentally have the signal going to them. Or, I heard Neumann makes a decent talkback mic.......