You can actually see the percentage by hovering you mouse cursor to that upgrade.

For my personal experience, radar thing is nearly useless on small maps, storage aswell. So the only viable option is either attack/defence or orbitals. But i think orbitals are far more profitable on the early stages of the game. You got to keep in mind that the larger your army grows the more benefits you get from upgrades (it affects more units, so more usefullnes). I often go for prioritasing attack, but you actually got to keep in mind that you can for example get additional 13% of attack for 200-500 quanta at high upgrade lvls (if you alredy leveled your attack), or get a 13% hp for 100 quanta for lower lvl upgrades in defence (if it is not leveled). Its all about tradeoffs.

So wait... you get more HP boost for less quanta?

How is that not a better deal?

Not quite. Its a question of tradeoffs. For example. For attack upgrade lvl 1 you need 100 quanta. Same goes for hp upgrade lvl 1. You get 13% of attack or hp depending on upgrade type. Each subsequent upgrade lvl costs more quanta but it ads roughly the same 13% of attack/hp. So for example if you got lvl 3 attack upgrade your dmg multiplayer is somewhat 137% (if im not mistaken). But to get additional 13% via lvl 4 attack upgrade you need to spend aroung 200-250 (dont remeber the exact numbers) quanta (cause the costs grow larger at every upgrade lvl). BUT if you havent leveled any hp upgrades, then leveling 1 lvl upgrade of hp will give you 13% of hp for your units, but for 100 quanta (because its a lvl 1 upgrade). So you must choose either you want to invest a lot of quanta in attack upgrade to get extra 13% of attack, or get 13% of hp for a much lesser ammount of quanta or even 2 upgrades on hp (for a total of 25% hp) for the price of 13% of attack. Same goes vice versa if you leveled hp first. 

It is possible to calculate the benefit for a single quanta spent. The deeper you go into upgrading a single type upgrade, the less profit you get for 1 quanta unit. Something like a diminishing return. So for best quanta usage on upgrades i would recommend leveling attack and hp upgrades roughly at the same time or with 1-2 lvl difference depending on your strategy.

Could be, i mainly play small maps, game ends long before i have enough quanta for this upgrade

Well as i said upgrades are mainly for mid game to late game. Early game it is far more profitable to invest in orbitals. But i do disagree that radar upgrade is useless. If you have a lot of high range units OR you need to keep an eye on enemies suprise attacks the extra radar range could be good. But i do wish devs would add another upgrade that included unit range of sight increse or add a bonus effect on radar upgrade so it would actually mater in small maps.