For our August 2019 trip - I booked Cabana Bay EARLY (like 5 months) because we were staying there regardless of price. Luckily, about 75 days in advance, there was a deal and I could re-book for the lower price. That's not a guarantee - but it does happen.
If you are staying at one of the hotels that offers the express pass automatically - you can't price it separately. Express Pass varies in price based upon actual days of travel. March is usually higher, than say a random day in Sept, on the Express Pass due to spring break traffic. That said - it is possible to book a non-Express Pass hotel and ADD the express pass for less than staying at a hotel with it included - but that all depends on the size of your party and where you are staying. But - that strategy is more work in terms of planning - and probably works better for a solo trip than say someone putting 2-4 people in one room and all getting the express pass at one of the hotels that's included.
I think hhn is Halloween Horror Nights - but I think you said you are looking at March.
I'd be REALLY careful booking any package for a trip to US/IOA. Be sure you really know what is included! I've seen packages that have perks or more park days than I was going to use - and I've never found a package at US/IOA that works for me. I've always found booking the hotel and getting the tickets separate to be best for me. I've bought tickets through AAA and Costco.
I also am REALLY glad I didn't buy tickets too far in advance for our last trip. They had a special on tickets - buy 2 days - get 3 days free! We were going to originally buy 2 days tickets - and that special gave us more park days - which was great for us.
When you see that much of a jump in price quickly - probably one night of the stay is sold out or nearly sold out. Play with your arrival/departure day a bit if you can. Or switch among the hotels. Or keep trying for a few days. Folks are changing things up all the time. I recently had a panic attack that I was going to have to change by Dec plans at WDW due to room prices (no value resorts available!) - but later in the day on my second day of trying - something opened up.
I don't recommend any of Universal's dining packages - I just haven't seen how they save you money.