-140

-100 because

Assuming 2 axes means 2 x axe, instead of being a different symbol in which case the puzzle can not be solved, or meaning axe x axe (or axe squared) which is sufficiently unlikely that I'm not going to bother doing the relatively simple math.

if you are using the proper algebraic order of operations where multiplication takes precedence over subtraction, the answer is -140

if you are incorrectly just doing operations left to right it would be -100

this is an ambiguous and poorly designed puzzle.

Andrew Adamson said:

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So there are 2 possible answers. 

Each helmet has to be worth 30 (30+30+39=90). 

Each pack of arrows has to be worth 5 (30-5-5=20). 

2 pairs of double axes could be either the value of one pair of axes added to another, or it could be the value times itself.  If added, then 5+(x+x)=25 so x = 10.  But if the 2 pairs of axes indicate that the value is being multiplied by itself, then we get 5+(x*x)=25 so x=sqrt(20). 

if x=10 then ?=10-(30*5)=-140.  if x=sqrt(20) then ?=sqrt(20)-(30*5)=~-145.53

so -140 or -145.53