Well, even with $200 for each thruster, the prices is still an issue for me when it comes to 8 thrusters ($1600)!!! 
But thanks a lot for the comments and your advices, it really helps me a lot. I like a lot the idea of a moving battery to change the center of mass, I think it is very clever! It remembers me of this project where pitch and roll where controlled like that: OSUG: Open-Source Underwater Glider | Hackaday.io
However this solution comes with the risk to get stuck in the vertical position without possibility to come back to the horizontal! 