Playing around with it a bit, really like it.
I hope we get more functions, like 'mult', 'add', 'sub'.
For now I will use stacksize selectors:
<configs>
<set xpath="/items/item[contains(@name,'resource')]/property[
@name='Stacknumber'
and @value > 900
]/@value">20000</set>
<set xpath="/items/item[contains(@name,'resource')]/property[
@name='Stacknumber'
and @value <= 900
and @value > 400
]/@value">2000</set>
<set xpath="/items/item[contains(@name,'resource')]/property[
@name='Stacknumber'
and @value <= 400
and @value > 100
]/@value">1000</set>
<set xpath="/items/item[contains(@name,'resource')]/property[
@name='Stacknumber'
and @value < 100
]/@value">250</set>
</configs>