DCL doesn't allow TAs to vary the price at all, so the price is the price. What you will see is some sites will include all taxes and fees while others make their costs look lower by not listing these till you actually try to book.
Beyond that, make sure you are comparing "apples to apples." Do the quoted include transfers, insurance, etc. or just the cruise costs?
Where TAs CAN differ is in the service they provide as well as any "thanks for booking with us" gift. As noted, this can be an onboard credit, a shopping card. a gift in stateroom, a coupon book, etc. Honestly, after excellent service, I'd prefer either a shopping card to a store that I already use regularly OR an onboard credit. The other stuff is just fluff--a coupon book is not much good if I don't use the stuff the coupons are for."
The only "hidden cost" would be your tips, which are $12 per guest per night of cruise. SO, if you did a 4 night cruise, the standard tips would be $48 per person in your cabin. The rest is obvious, but needs to be considered. You have to get to the ship--that's likely to be a flight with transportation from the airport to the ship, but could involve driving and parking your vehicle....a flight could involve a cost in getting to your home airport and possibly parking there.
If you arrive the night before, decide first where you want to stay. There are hotels in the airport area that offer a stay, breakfast, and transportation to and from the cruise terminal. You can stay at a hotel IN the airport and ride the DCL bus in the morning--this option is very convenient but also costly. You can also choose to stay at a hotel near the port in which case you need to rent a car or arrange a towncar/limo/shuttle service to get there.