It is the context that matters for these words in German. There are no hard rules really noted - so the following comes from experience.
In the examples I use a valve (Ventil) in a radiator (Heizung).
Is used when I get a part that is exactly like the one that was there. For example to replace an AAA battery that is empty with a new one. Same build, same function.
Most often used in the context of getting the same item as before and in repairs.
As with "Ersatz" (replacement) the action "ersetzen" implies some item that is no longer working / functioning as expected.
"Das Ventil in der Heizung leckt. Kann das ersetzt werden?"
"The valve in the radiator leaks. Can this be replaced?"
Is used when I want to exchange something. Rather even more restricted than "ersetzen" for items as used during guarantee replacements. Note that this can also mean I can get a very different item than the original purchase as the replacer sees fit.
"Das ersetzte Ventil in der Heizung leckt schon wieder. Ich fordere einen Umtausch."
"The replaced valve in the radiator already leaks again. As it is under guarantee I expect a replacement."
While this word can be used for both the previous words (and will be understood by Germans after a bit of thinking) it can also be used in a broader context.
From the verb "austauschen" its meaning is the replacement by a functionally equivalent item. Not necessarily the same. Implies also that the original item still functions although for whatever reason.
"Das Ventil in der Heizung ist schon ziemlich alt. Kann das ausgetauscht werden?"
"The valve in the radiator is rather old by now. Can this be replaced?"
In context of language either "ersetzbar" or "austauschbar" applies if one word (or phrase) can replace another without losing the context.
With regards to money the more common word used by Germans is "bezahlbar" (payable). The monetary phrase "is exchangable for" in German becomes more often "hat den Besitzer gewechselt" (has changed owner(ship)) or "hat einen Wert für" (has a value for).