No one wants to spend much for anything! Everyone has a different opinion as to what is too much, so it is best to determine the customer's perception of too much before starting. It could be obvious that repairs will be more expensive than they are willing to spend.
I want to spend only enough to find out what is wrong, and what it will cost to repair the necessary thing(s), and items directly associated with those repairs. If there are optional items that do not limit the functionality of the equipment, I'll consider them as a separate cost. If the necessary repairs are not too expensive, based on the age and condition of the equipment, then optional items should be deferred until the critical repair outcomes are known to be successful. Of course there are always exceptions, but you have to start somewhere. The condition of the equipment after repair is also an important factor, because items in obviously poor condition might soon cost more to keep the equipment going.
The rule of thumb for industrial equipment is to limit the cost of overhaul to 50% of the cost of a new replacement. There are also exceptions to this too, like what new offerings are available that offer better features, etc., and the lead time for acquisition.