I got a bit further: it because "{!isAccountSelected}" is false when the page is loaded. If I set it to true at the start, it works normally.

I added that tag but I get no errors whatsoever.

Nope. Nothing happens, no errors, nothing. It just doesn't go into my Apex reference function.

Well, a debug message appears, but my System.Debug string isn't listed.

@PhilHawthorn awesome, thanks! didn't even see it :P, it was at 110%

Thanks for your comment :) I tried that but it doesn't work. It will just disappear at some point then?

@AlexTennant blugh it seems wrong to make such a arbitrary work-around. But it works ^^

but what if I can't reference the value I want to sum, because the budget of a project is calculated using an employee salary which is an outside object (apparently I can only reference formula's that are calculated using 'local' values)