Calculate checked bag fees.
Bag fees are tiered rather than flat: the second bag typically costs half again as much as the first, and the third more still. Overweight surcharges are the sharper edge — commonly a hundred or more per bag, charged at the desk with no way to avoid it, and often more than a second bag would have cost. Every one of these charges applies per direction, so a round trip doubles the lot. Redistributing weight between two bags at check-in is usually the cheapest fix available.
Checked bag fees
Per direction = first bag fee + (bags - 1) x additional bag fee + overweight bags x surcharge; total = per direction x 2 for a round trip
Usually, and often by a wide margin. Where the additional bag fee is below the overweight surcharge, splitting a heavy bag into two compliant ones saves money — worth checking before you get to the desk.
Substantially. Pre-paying at booking is typically 20% to 50% cheaper than adding a bag at check-in, and airport counter rates are the highest of all.
Not always. Fees follow the operating carrier and the fare's own baggage rules, so a return on a different airline or a basic fare can cost more. Run each direction separately if the carriers differ.