Try dumping a quantity of the supermarket stuff into a non-corrosive sauce pan and reducing it at a simmer by 25%. The expensive stuff has been aged in barrels for many years. As it evaporates off through the porous wood it is moved to smaller barrels, (or combined into other barrels.) The aging process mellows the acidity and concentrates the flavor.
Reducing the vinegar as I've suggested will give you greater depth of flavor than the add-ins you've been trying. Re-bottle the reduced balsamic into an appropriate cruet etc. Reduction will increase the cost of the end product, but not that drastically. Play around with how far you reduce the balsamic. You can reduce it all the way down to a syrup consistency and it will never have an unpleasant flavor until it begins to caramelize. The reduction process does not seem to affect the overall acidity experienced. To make syrup, reduce the vinegar about 80%-90%. A little syrup goes a long way. Balsamic syrup is great lightly drizzled on grilled fish served with roasted or grilled potatoes. It is also nice drizzled into some decent olive oil for bread dipping, (it produces an interesting lava lamp effect.) You can get good value, large bottles at costco or trader joes that will work fine. If you have a friend in the restaurant business they can get you 5 liter jugs that will work great for about $40.00. Even reduced in half it will be far, far cheaper than fancy little bottles. It will lack some of the subtle nuances found in high end products, but I am sure you will be rewarded if you play around with this proposal.