Thought that this one is kinda obvious, but appears that it ain't
I'll use attack as it matters to me most, but you can do it your way.
So you buy one of each RP items that interests you and notice the gain each one gives. Then divide given items cost by the gain it gave you.
For example I gain 16 attack per RPG, 23 per exelero and 18 per Whip.
RPG. 1000 / 16 = 62.5
Exelero. 1200 / 23 = 52.173913
Whips. 750 / 18 = 41.6(6)
Obviously, each point from chain whips costs less so those are the ones I go for. As soon as you start getting less from your item, recalculate and calibrate.
Boom blam wham! Yada