I've read that too, but mostly here. I've been to Belize a few times and it doesn't strike me as expensive. (Granted I'm not real price sensitive, but I also like to get what I consider a decent value.) There's a very wide range of accommodations and prices. For your midrange nice-enough experience it seems pretty reasonable and you could do it pretty cheap if you wanted. Maybe more than Honduras, but cheaper than many other Caribbean destinations. I think you just have to look up some places you're likely to stay and dive or whatever, and decide for yourself according to your own definition of "expensive."
There's no shore diving, so you'll be paying for boat dives. That's expensive if you do longer trips like to the atolls (and not worth it IMO if you're mostly going out there for the Blue Hole.) But boat dives off say Ambergris aren't far and look fairly reasonable (again compared to some other places.) Land tours can be what some might consider expensive, but after doing them they always seemed well worth what we spent. Some of the $ is related to distance, you can do a lot of traveling to get to things in Belize.
From my location, I've generally found better airfares to Belize than Honduras.
