I used to play around with teleconverters, especially when I worked in the
trade and could borrow stock.
I can honestly say that the general purpose ones (even the expensive ones at
that time) were pretty poor.
The "extenders" you get for specific lenses did work but the cost of the
extender made it a better bet
to go the whole hog and have two lenses.

It really depends on whats available second hand or discounted new at the
time you go looking,
or you could do what I do and spend years lookig before pouncing on the
right lens when it comes along.