I know back in the day surface receivers wouldnt work with air receivers as they where on different freq's. Is this still the case with the new 2.4g stuff?

Reason being I am looking for a cheap 2nd receiver for my new MT-4 for a rc18t I had laying around and I really don't want to spend $70.00 for a 2nd cheapo receiver when I can get a complete radio for almost that much.

I see that the air 2.4g receivers are a little cheaper. Is that an option?

Thanks.