Just from experiece (i.e. observing combat) weapon/shield/tower upgrades offer diminishing returns on the buck spent.
As a general rule, at least for ship-to-ship weapons, damage seems to upgrade linearly, like basic laser seems to be 1x damage, photon 2x and plasma 3x, but the cost will rise more exponentially. Same with big asteroid turrets, although they go Laser->Plasma->Photon.
I'm not sure about the exact damage steps, they may not be exactly 1x-2x-3x, but at the very least plasma seems at least 2x more powerful than laser for ships, in fact probably more.
Antisurface ship weapons are a little trickier, but also come with diminishing returns for money. However, some of these weapons can prove more effective against densely populated asteroids where they can hit multiple buildings, particularly napalm and especially vortex. On a densely populated asteroid Vortex is probably the most damaging weapon, due to the length of time it says active and napalm can fry adjacent buildings slowly. Chaos bombs on the other hand have absolute best alpha-strike factor and and single target damage, which may be crucial.
There seems to be benefit in doing weapons mix for antisurface operations, for ships though it seems the idea was that player will upgrade to more powerful as cash becomes more abundant.
When it comes to choices and blueprints, at least for me, cheap and dirty upgrades seem to be quite beneficial for the early game when cash has to be preserved for expansion, - I generally get the plasma turrets and shield 40 very early.
So my early defences would be 2-4 those plasmas and a small fleet of about 12 combat eagles with photon and 40 shields.