There are a a few different types of vinegar that are sold as Balsamic Vinegar.
According to McGee in On Food and Cooking:
1. true Aceto Balsamico - which can be upto decades old, with a consistency of treacle and aged / concentrated in wooden casks. - and is very expensive. usually labeled "Traditional"
2. Condiment grade balsamic vinegars - are made more quickly and mass produced. Usually from cooked down Grape must and some young balsamic vinegar. These are suitable for a salad.
3. Cheap "balsamic vinegar" - is often no more than standard generic wine vinegar coloured with caramel and sweetened with sugar.
Best I have used and tried..but not too pricey I have to order online. It's the Balsamico of Castello di Gabbiano (from Tuscany). I think it has the right balance of tart and sweet. I tried many purchased locally from Whole Foods to Trader Joe's to Surfas...and nothing compares. It costs about $30 a bottle.
Good balsamic is like good Scotch, Port, Cognac, Bourbon, wine, etc.. The best stuff should be reserved for that special dish where premium ingredients like these stand on their own without combining with flavors of other foods. They should enhance the experience without dominating it.
I don't use Balsamic at 3.5 ounce = $500 levels. That's waaaaay over my head. But I do often find a good balsamic below the $50 for a 5 - 8 ounce bottle. Be sure to avoid anything sold as "Balsamic Vinegar and ....." (adulterated with things like herbs or honey). If the balsamic isn't good enough to market on its own, adding things to it seems to be a popular way to get it out of the warehouse.
Also keep in mind that there are no regulations that control the labeling of vinegar using the term "balsamic vinegar". What you purchase, regardless of what the label says, may or may not be balsamic.
Partly because of that fact, I prefer to use imported balsamic vinegar. Balsamic vinegar originating in Modena Italy is a favorite.
You need to give us a price range of what you are comfortable spending. Traditional Aceto Balsamico can cost as much $500 (or more) for 3.5 ounces.
You also need to tell us what you intend to use the vinegar for. Incorporated into dressing? Cooking sauces? On it's own (e.g. over ice cream or paired with fruit)? I would never use expensive aged Aceto Balsamico for things like salad dressing or making sauces, but would reserve it for pairing with fresh peaches or maybe drizzled over roasted cabbage or asparagus.