- Are there different qualities in prepared Hoisin sauces?
Think of it like barbecue sauce or chili powder: every company has their own recipe, and it's always hard to know the precise justification for a specific price. The high price on one may be part of their premium brand image, and have nothing to do with the quality of the ingredients. Ultimately, you just have to try a few brands and decide which flavor appeals to you, and whether you find the price reasonable.
- If so, would I be better served by making my own?
As far as I can tell, hoisin sauce does not need to be fermented, so it's going to be much easier to make authentic hoisin sauce at home than something like soy sauce or Worcestershire sauce. It's more like making your own barbecue sauce.
A few different websites have minor variations on this recipe: http://www.cdkitchen.com/recipes/recs/420/Hoisin_Sauce12082.shtml
Is it worth the effort? That's a matter of personal opinion. It's probably hard to justify on purely economic terms; do you really use more than a few cents worth of hoisin sauce in any given meal, and are the ingredients that much cheaper than the product itself? But of course, there are other reasons to cook than just to save money. I think you've just inspired me to try making my own hoisin sauce :)
- If I make my own, how long will it keep for?
Each of those ingredients can last months on its own, often without any refrigeration, and together would create a highly acidic and antimicrobial environment. The worst that might happen to homemade hoisin sauce in several months of refrigerated storage is it might separate a bit and have to be remixed.