I am not a travel agent but I have a similar exp that I can share. The price for my cruise (in 2 weeks) dropped $400 total as of yesterday. I had a 8I gty room for the 3/3 sailing of Spirit. I called the TA and she called Carnival (after a lot of hesitation) and only offered me a $25pp shipboard credit. She explained that this was because we were within the penalty period (70 days prior and paid in full) and when price adjustments are made, Carnival (since 9/11) actually cancels the reservation and rebooks you. Sure you get the dropped price but you will have to pay a $250- penalty per person, so most of the time, esp. with my case, it is not worth it. I didnt like this explanation at all (esp. since I was now angry at my TA for never telling me this). I called Carnival myself and spoke with a very helpful rep., who spoke to her supervisor and I was upgraded to a 9A with a cabin assignment. I wasnt thrilled with that either but my TA couldnt even do that so I took it. It was a take it or leave it kind of deal. At home, I checked the prices of 9A and we arent getting such a deal on that either now. I would have rather had the money back. I did call Carnival that night after seeing this and the rep. who answered refused to spk to me claining I booked through an agent (why did the first one talk with me)?! Anyway, I say, look forward to the trip, never check the prices if you know you will get angry about drops in rates or check and hope you ghet the right rep to deal with.
So, the 'real' answer to your question is, depends on who at Carnival you talk to. The 'technical' answer is no price adjustments within the penalty period (which I think is totally unfair).
Speaking as a TA, the answer to your question is supposed to be "NO". Once you make final payment, you are within the penalty period and you are no longer a threat to cancel, unless you want to pay the penalty. Therefore, a cruiseline (and I mean all cruiselines) does not feel compelled to appease you with price reductions. That being said, there are variables that will change that policy:
1. If your TA is a high volume seller for the cruiseline you're trying to get a post final payment refund on.
2. How hard your TA goes to bat for you.
3. If the ship is still not very full.
4. If you get an agreeable cruise line rep when you call.
Because of this, it is still very up in the air how you would make out. It never hurts to try and if you can't get a refund, try for an upgrade or shipboard credit.