280

Helmet = (30+30-30)= 30
Quiver = (2Q+30=130) = (2Q=100) = 50
double axe = (50 - double axe -30 = 10) = (20 - double axe = 10 ) = 10

So if single axe = double axe /2 = 5 answer is 30+50x5 = 280
but in reality we don't know the value of single axe

If helmet + helmet - helmet = 30, then 30 + 30 - 30 = 30, so helmet = 30

If quiver + quiver + helmet = 130, then 50 + 50 + 30 = 130, so quiver = 50

If quiver - double axe - helmet = 10, then 50 - 10 - 30 = 10, so double axe = 10

Therefore helmet + quiver x single axe = ?

If double axe = 10, then single axe = 5

30 + 50 x 5 = 400, so ? = 400

400 is the solution to the puzzle