Since you are on 72mhz...

Polk Hobbies has their Tracker III 72 mhz transmitters on sale for $99.00.

These can handle up to 99 planes, have computer controlled mixes, will operate on any 72mhz frequency without crystals, have a built in frequency scanner, are shift selectable ( works with JR, Airtronix, Futaba, etc. receivers! ) has 8 channels, comes with a servo, receiver & transmitter battery pack, switch, etc. etc. etc.

I've been quite impressed with these and I just purchased a second one to use with my planes that are on 72mhz and have their own synth receivers.

Since I can change the frequency of the transmitter ( and the receiver ) at will, I'm less inclined to spend the $99 times 30 some planes ( $3,000.00 !! ) to switch my entire fleet over to 2.4ghz.

I've been switching my newer larger planes over to 2.4ghz slowly.