yeah, Young Island definitely costs more than the other options, but the other places aren't in anywhere near the same league. It's a small private island resort with all sorts of unique features, far from your run-of-the-mill hotel. (Actually it's prices are a lot less than most similar private island resorts you'll find in the Grenadines, and I don't think good diving it so convenient from any of the others. I've even done a couple shore/muck dives right off the beach.) The rooms are in stone and wood cottages, all different, surrounded by dense tropical foliage with outdoor showers - great to wander straight in there in your wetsuit.

Level of service is high, and meals are fine dining that you don't really have to dress up for. Unless the weather is really bad meals are served in little thatched roof gazebos on the beach, private or just a few tables in each, with peacocks begging at your table rather than the usual dogs/cats/birds. You have a whole island to explore, fun to check out all the little trails and views and stone construction. It's just a little paradise.
The cheapest rooms are along the water or beach, the cottages get bigger and fancier as you go up the hill, with views (good) and climbs (bad) to go along with it. Works for me - although those cottages up on the hill look pretty neat, I'd rather stay at sea level and not slog up and down for diving, meals, swimming, drinks, everything. IMO "nice place to visit wouldn't want to live up there" applies.
All that said, for a somewhat more traditional hotel option for less money, from what I know I'd probably pick the Mariner, just across the channel on the main island. But Young Island is part of what makes us like this particular trip so much - it's hard to find places that combine luxury and great diving well.