On our way to developing a satellite, we've used balloons to test hardware and software. We've also been experimenting with solar balloons, a type of thin-plastic balloon which doesn't require helium (which is in short supply, worldwide), but instead utilizes heat from the sun to generate lift.
We've developed a 3D printed CubeSat-like structure as well as a cut-down mechanism to terminate the balloon's flight. We're aiming to make a ‘BalloonSat' kit that can be constructed by schools for between $50 and $150 (depending on the desired types of hardware included). Papers related to our ballooning efforts can be found on the documentation page.