Expression for losing something that you never really had

Notional loss?

By not buying a lottery ticket, one could "lose" a million dollars.

As opposed to a real loss of a dollar or two by buying one.

This answer and the OP's example may not seem to match. That is because in the example, the loss is real, and so the example doesn't quite serve the purpose of the question.

( meta: For some reason, I'm unable to post comments for now.
A nominal loss is a symbolic loss or usually, an insubstantial, so negligible, loss – but real loss. It's not hypothetical like notional loss. )

In the trading world, we'd call this a paper loss, and its theoretical nature makes us a bit cynical and snarky about it.

Of course, a "paper loss" has its counterpart in "paper gains", which we're even more cynical and snarky about. The whole idea is encapsulated in the term "paper trading", whose Investopedia definition reads, in part:

As a result, basic investment strategies such as buying low and selling high – which are quite difficult to adhere to in real life – appear relatively easy to make while paper trading. The first lesson of paper trading, therefore, is that while it is a great practice tool, it is very different from actual trading with real money.

Your title, and description don't match; the title is more interesting to me so that's what I'll strive to answer.

Lost-Potential:

According to Merriam-Webster a definition of potential is:

existing in possibility

• Ref: http://www.merriam-webster.com/dictionary/potential

and a definition of lost is:

no longer possessed

• Ref: http://www.merriam-webster.com/dictionary/lost

So perhaps lost-potential would be a good term?

lost-potential:

Loss of something not yet gained.

Delusional-Loss:

According to Merriam-Webster a definition of delusion is:

a belief that is not true

• Ref: http://www.merriam-webster.com/dictionary/delusion

and a definition of loss is:

failure to keep or to continue to have something

• Ref: http://www.merriam-webster.com/dictionary/loss

So perhaps delusional-loss would be a good term?

delusional-loss:

A fictional loss.

Fictional-loss:

According to Merriam-Webster a definition of fiction is:

something that is not true

• Ref: http://www.merriam-webster.com/dictionary/fiction

and a definition of loss is:

failure to keep or to continue to have something

• Ref: http://www.merriam-webster.com/dictionary/loss

So perhaps fictional-loss would be a good term?

fictional-loss:

an imagined possessional adjustment

If you were to relate the SE Rep system to a more traditional concept, like filling a cup with water, the cup can only contain X amount, and the rest is 'wastage' or 'overflow'.

'overflow' has the potential to be recovered, but 'wastage' is more final, like the loss from the cap. But 'wastage' implies carelessness, or negligence.

This brings it more in line with the concept of 'overkill'.

overkill: something that is much larger, greater, etc., than what is needed for a particular purpose

Their answers were so good, that there was a +60 overkill

That works for me.