Do I need an expensive balsamic vinegar?
While using an expensive balsamic vinegar may give you a deeper flavour, it's not essential.
As you need quite a lot of vinegar to make just a few portions of this it can be really pricy to use a superior quality bottle. Spring for the best you can afford, knowing that you will be pouring a lot of it in a pan to boil down.
Cheaper vinegar may also be tarter in taste, so again, while it might be more economical to use a cheap bottle. Opt for something mid-range if possible.