Great idea Earl!
How light are you aiming for? For a really light payload a 3DR 433 radio wired straight to a MTK GPS would be about as light as you could do. Just set the radio to 38400 serial and it will pipe the GPS data straight from the MTK to the ground. No need to have an APM at all. Just apply 5V and you've done.
An APM would be nice for the barometer of course, but it will add some extra grams.
What sort of balloon/payload are you thinking of?
btw, we still have 12k of flash in the radios SoC. So we could make a special 'balloon flight' firmware that would send extra data. For example, the radio has two temperature sensors builtin, so might as well log the temperature and send it over the radio link (the latest git code uses the Si1000 temperature to prevent PA overheat on a radio that another diydrone member is building).