[Alneyan]

You do not get the Merchant culture for free, as it means foregoing the Berserker culture. Merchant gives the rough equivalent of 1,000 points (if you have 110 maintenance or more), while Berserker gives +10 in both offence and defence, a virtual bonus of 2,000 points if you were already at 120 or above (and you should not be anywhere lower than 120).

The reductions are more or less the same for both cultures (mainly minerals and research for Berserkers, space combat for Merchants), but while going above 120% Maintenance is useless, Berserker is the only way to have Aggressiveness/Defensiveness above 125, making this culture the most appealing of the lot. Merchant is, of course, a strong second, along with Warrior.