The same can be said for you! And I'm sure you cost a bit more than 4gold a month to upkeep.

In all seriousness, Sea Trolls/Trolls are fine upkeepwise. There has to be a reason why you wouldn't use them every, single time. And the reason is that they require you to be on the offensive and/or in active attempt to destroy them by some means.

If you can't tell, I like Trolls of any flavoritity and use them often