Wow, cp has nothing to do with iv, iv just influence the max cp you can have.
So if you find a 100% iv, upgrade it, it will cost you a lot if he is low level (so low cp) to upgrade him, so better find a 100% with high cp so it will cost you less stardust.
But 100%iv are very rare, so if you find one, you can upgrade it he will be '' the very best '' the cp only influence on the stardust you will need to pay.
Upgrading a pokemon = leveling up, and a pokemon can be max lv 31.5 if you are lv30 trainer.
In other words, iv is like the genetics of the pokemon (ex: you get a big dick as a kid, you will keep it all your life) and the cp show you the level of your pokemon, and if he is lowlevel you will need to pay a lot of stardust to grow him up to your trainerlevel.