I purchased a Radio Shack wireless unit a year or so back and was not satisfied. I returned it. I don't recall any of the details, but knowing my mind, I'd say it cost under $50. So it wasn't a high-tech unit I'm sure. Later I purchased a pair of Cobra "walkie Talkies" and they work great, even over a distance well outside the house. If they came with separate chargers, that would be perfect, as one can then leave them sitting in the base and "ON" all the time. This is easy and affordable, but to date I don't have a second charger so they are not used for the purpose. The perpose of my post is to share my experience, namely wireless can work, the Cobra WTs work. Good luck.

Wireless intercom systems are great! They are effective and very much reliable. I suggest that you buy one with a range of up to 1,000 feet for a wider coverage.

Here is a link that might be useful: Guide to Wireless Intercoms

fwiw
I recently bought some Philip's 1.9Ghz cordless phones. Less than $100 for three, one with an answering base and others with charger bases. They do a sort of intercom.. as do some of the other cordless phones. Philips you push conference button and dial 1,2, or 3 , and it rings just that phone.
Think GE's, basic type of 1.9Ghz DECT phones, might do hands free intercom.