I live in a city with many international ferries departing each and every day.
Here the prices changes all the time and you should book the tickets as soon as possible. Also the prices are different (much higher) if you're booking using the English/international version of the website.
However the prices I've seen for the Ancona-Split ferries seem pretty fixed (I've looked at SNAV and Jadrolinija). Are they not changing much?