Rush building costs 15 credits a manu point, which makes it a horrible investment. I'd reduce it to 10 and see if it is still too high.
Upgrading is also insanely expensive - something like 240 to upgrade a tiny scout (I had 4) to use 3rd level sensors, life support, and engines, and 760 to upgrade the small explorer from 2 to 3. I'd reduce the cost of upgrading Sensors, Engines, and Life Support by half compared to the cost of upgrading weapons and defenses, and even then probably drop the costs of upgrading by a third, proportional to my suggested reductions in rushing.
Having to run your whole empire at 100% wealth generation for several turns to upgrade your survey fleet is silly. Doing so during a war to upgrade your combat fleet, sure, but not your scouts during peace.