If you want to be able to get the full use from the Sony TTL wireless system, which I occasionally opt for over the radio triggers, I'd go with at least another 42. However, if you want another off camera radio flash at a great price the Lumipro LP160 is dang hard to beat:
Midwest Photo Exchange LumoPro LP160 Quad Sync Manual Flash
The Cactus V5s are out and have some advantages over the V4s, or check out the Phottix Strato II Sony triggers so you don't need any adapters with the Sony cameras or flashes.
Phottix Strato II Multi 5-in-1 Wireless Trigger (Sony ones shown in last photo. I have a Transmitter/Receiver set and am pretty impressed with them, and they can wirelessly trigger the camera with the included cable.