What is the word or saying for when you pay money for something, but don't end up using it?

Not a word, but an idiom: "it's all been money down the drain".

• to waste money; to throw money away.

literally it means

From the web:

"Don't buy any more of that low-quality merchandise. That's just throwing money down the drain."

"Now, as police destroy the 200th bike they have confiscated, officers are warning parents not to pour money down the drain by buying the bikes for their children this Christmas."

I'd call it a waste of money, and it's a very common expression.

(noun) money spent for inadequate return
"the senator said that the project was a waste of money"

^TFD

a bad use of money. "The show was a waste of money."

^M-W

And a person who wastes money this way may be called a spendthrift.

Your examples focus on buying services. For the case where you buy an object small enough to fit on a desk, counter or table, you could refer to it as an "expensive paperweight." You could use this expression if you bought something that was reasonably expensive but then you ended up not using it (because it didn't work well enough or didn't do what you thought it would do), it broke after a short time, or something else happened that made it unusable, useless, or obsolete.

expensive paperweight

An object that may have had an original purpose (usually electronic devices) or a concept model of something else, but in any case unable to serve its intended purpose except as a display item or a paperweight.

Since many of these paperweights, especially those of the old electronics category, required a larger investment at its initial purchase, these paperweights become expensive, hence, the "expensive paperweight." (Urban Dictionary)

For example:

This hack seems to have turned an iMac into an expensive paperweight.

Turning Classroom Tech Into More Than an Expensive Paperweight.

[Your textbook] is not just an expensive paperweight.