Yes, they will, but don't expect them to be watching the prices. If you find it goes down call immediately and inform them. They will then get on the horn to Carnival and have the cost adjusted if possible.
As long as you're outside of the final payment date your TA will be able to give you the lower price. If you're inside the final payment date you may get some OBC or an upgrade, but don't count on it!
As tribbs1 said, you should watch the prices yourself. Most TA's will check prices about once a week, but you may find a better price by checking the Carnival site yourself and letting her know!
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