Buying Items - Spend Gold Efficiently by BlueBadger

Just gonna copy a comment I made on your more complete guide, since it is specifically relevant here!...
About the cost of each stat on items and the value (or cost, rather!) of passives and actives on items, here is what I found:

For example, weapon power (wp) seems to cost about 19 per point. Tier 1 items have a "discount" of about 70 gold, tier 3 items cost on average 338 more gold than what their stats are worth. (One tier has to be the baseline, I found it logical to interpret in terms of tier 2 being the baseline.)
You discussed the value of Tension Bow's passive. I find it to cost 490 ("tb" in the table) rather than the 750 it costs over the 2 items required to build it. There is a difference mainly because of the inherent cost of a tier 3 item (338), independent of its stats, and also the "fact" that its prerequisites are not necessarily priced at the average cost of their stats.

(I used the following non-obvious acronyms: sp=spell pierce, ap=armor pierce, cpls=cystal lifestyle, wpls=weapon lifesteal, critc=crit chance. And for passives/actives: fb=frostburn, ac=alternating current, as=aftershock, bm=broken myth, hpk=hp per kill, bs=bonesaw, bp=breaking point, tb=tension bow, rb=reflex block, rbt=the 2 reflex blocks you give your Team or the difference between the passives of reflex block and crucible, ls=lifespring, for=fountain of renewal, atp=atlas pauldons)

I did not take into account energy and critical damage%, because they are highly correlated with energy recharge (recharge) and critical chance (critc), and including them messed up the results/interpretation without helping to predict the cost of items. I also did not take into account utility items, because there's not enough of them to get any meaningful result by this method.

I suspect that there are some non-linear relationships which could help explain even better the cost, however, this "model" already explains 99.89% of the variation in costs and adding weird variables would have too many downsides due to the small sample (number of items in the game).

One thing I found interesting is that it seems the specials on frostburn, crucible and fountain of renewal do not cost much, which encouraged me to use these items more often than before!

This is great stuff! I hope to see more "nerdy calculation" guides in the future! Upvoted.