I agree with Chilly.
When people ask whether the Blue Hole is worth it, usually they're referring to the time and effort it takes to get there, not the money.
Time and effort is an easy one. It's not so much just the Blue Hole, it's the entire day that makes it more than worth it. The ride from the Blue Hole to Lighthouse Caye is spectacular. LH Caye itself is really cool. You feel like you're at the edge of the world out there. It's a wonderful place to have lunch and spend a couple of SIs. And of course, there's the Red Footed Boobies Sanctuary as a bonus. The remaining two dives in the area are always fantastic. The BH dive is just the first dive of a pretty great day.
Worth the money is harder because it depends on your priorities and budget. Generally speaking, if you can afford a dive vacation in BZ you can probably afford a day trip to the BH. On the other hand, some people really do vacation on a shoestring and have to carefully consider each dollar. But chances are you would have spent money doing something else, so you could subtract that amount as a sunk cost from the cost of a BH trip to figure out the difference, which probably wouldn't be very much.