I would spend a reasonable amount for an on-board airspeed indicator. Some sort of sound interface would be the way to go. A whistle that decreases in pitch as your speed decreases with a beep close to the stall speed is a good idea.

I guess the weight, battery draw, and complexity of putting a telemetry transmitter onboard might be unattractive except for larger scale planes. That and the added cost of having a receiver in your transmitter case with a diplexed antenna.