First question I have to ask; why did you get a check? The final payment is not due until December 31st and making full payment before this is usually not advisable. Plus, you should ALWAYS use a credit card when making a purchase like this for several reasons. And if you booked through an agency, you always want to make sure that they did not charge your credit card - they should have the cruise line charge your credit card.
As for pricing going down, it can just go down because the sailing is not selling real well and they need to stimulate sales. Same thing the other way - if it's selling real well, then prices could go up. But it's always a good idea to book early because if they prices go down, you can usually get the lower rate provided final payment has not been made. If the prices go up, then you're protected against any increases, unless taxes or fuel supplements are involved.
I don't know where you live, but currently there is a special resident rate being offered on your cruise. So you might want to check it out and perhaps you're due for an even bigger refund. Keep your eyes open, anything can happen.
But I have to say, I'm concerned about the part of someone sending you a check. I've not heard that one before, so I would be curious to find out how you booked this.