Your goal for a TA OBC should be 5-8% of the fare (not incl tax & port). For a $25 or $50 OBC (for a 3 or 4 day cruise) I just usually book direct as I figure it's barely worth their time. If it's a longer cruise or yacht club I use a TA because the $150 + in OBC covers the auto grat/hotel service charges. However I will say that I do 100% of the work for the TA, literally spelling everything down to ship, sailing, cabin, etc.