Sooo hard
I tried to handle with AC.
While speaking of pure AC things are quite clear.
I created a formula for AC and tested it on robes.
For -0.6 robe i've got price of 405 drachmas.
For -0.7 robe 1280 drachmas.
For -0.8 robe 3125 drachmas.
For -0.5 robe it adds 80 drachmas.
For -0.4 robe it adds 5 drachmas.
For worse AC robes it should be substraction from price for AC penalty, but i'm not sure in numbers.
But, trying to handle other bonuses it become much more complex.
I tried to calculate price for -1.4 helm with my formula, considering price of 250 for max_stat and 100 for -saves.
Also, i was considering the difficulty of getting ingame eq (well, manually, hard to deal with it too).
So, best AC ingame helm is -1.3 +max_con -saves. Difficulty of getting it is quite low.
I got a price for -1.4 helm = 95 drachma.
It looks reasonable for me, because i won't trade my robert's hat to -1.4 even for 95 dr.
I calculated a price for -1.5 helm and got 970 drachmas.
Well, it looks... interesting. May be 970 is even too high price, not sure.
The problem is that ingame helm is too good.
I'd say this +max_con should be not +250 drachma, but multiply to AC price somehow.
There is huge difference between -1.2 +max_con and -1.3 +max_con or -1.2 +max_con and -1 +max_con.
The value of max_con changes high because of AC.
So, for -1.3 helm the value of max_con might be more than 250 and for worse items it could be less than 250.