There is NO travel agent markup. There is travel agent mark downs, if they want to reduce the cost.... and many do.
The cost from the cruise line, is the same for a TA as for you. Only difference is the TA's commission is included in the price, If you book through the cruise line they don't have to pay the agent, but you DO NOT benefit from that savings, the cruise line does.
If a travel agent is asking for more than the cruise line price, you've most certainly stumbled onto the wrong agent.
Kuki has the correct information. Actually most TA's will try to meet or beat internet offers, if they can by offering group cruises usually. TA's will usually throw in some perks like shipboard credit, photo vouchers, champagne etc. Your local TA wants and needs your business. If your TA charges you a service fee go somewhere else.