Either that, or there is one too many ) immediately before the && 🙂
Removing some parentheses might make it a bit easier to read:
if( $own_minrent + $own_maxrent - $minrent + $maxrent <= 100
&&
$own_minrent + $own_maxrent - $minrent + $maxrent >= -100
)
I also took the liberty of multiplying through by 2.
Actually, now that some of the undergrowth has been cleared away that looks like it could be simplified further
if(abs($own_minrent + $own_maxrent - $minrent + $maxrent) <= 100)