All right, so the first things I had to think about were the following:

• What do I want to measure?
• How big do I want it to make (for what class of rocket?)
• For what price?

For the sensors, I settled for three measurements:

• Acceleration
• Temperature
• Height (calculated from the atmospheric pressure)

The acceleration will be measured by a MPU6050, pressure and temperature by a BMP180.

I'll throw in an Arduino for the processing, a SD card reader for logging purposes, and a small radio module as a beacon (and maybe later telemetry...)

I want the whole thing to fit in a small-sized model rocket (C or D class engine). I settled for a BT-55 size, which uses a 32mm inner diameter tube. I'd like the assembly to be able to slide in and out of the tube easily.

My initial budget will be around 20€, we'll see later on the total cost... I'll mainly use chinese breakout boards, to reduce the costs as much as possible...