From the Internet, I've found a few data points:
1) "I don't care. No need to return money to me/us. Keep the money. Do whatever you want. Good luck."
- Seems Y Combinator and some good investors go this way [4]
2) "I'll sell our shares back to you for $1."
- KPCB sold their shares back to Gumroad for $1 [1]
3) "Return my/our original investment back. 1x return. No need to worry about inflation over the past 5 years. Just return the original dollar amount. Let's both move on."
4) "I'll sell our shares back to you for 120% ~ 500% returns. Pay me/us back within X months using your cashflow or just borrow money to pay me/us back."
- Buffer used cash to buy back investor shares [2]
- Wistia raised debt to buy back investor shares [3]
If you are an angel/VC, what did you do?
[1] https://sahillavingia.com/reflecting
[2] https://buffer.com/resources/buying-out-investors/
[3] https://wistia.com/learn/culture/taking-on-debt-to-grow-our-own-way
[4] https://share.transistor.fm/s/43dad61e
However, the reality is that you may have gentleman's agreements or reputational concerns to uphold. "Intentionally bad-acting" vs "failed" founders will be blacklisted from one or a cluster of funds. Sometimes there are other concerns. For example I have, due to informal agreement, personally returned capital to small scale investors in past ventures.
In an extreme case, I have even seen investors screw over inexperienced founders demanding they work to pay back debt and that they "owe" them the capital invested, even after a venture collapses. This is absolutely unfair and not the sort of thing founders should put up with.
I would encourage anyone asking this question to educate themselves as to the standard mechanics by taking TechStars' course Venture Deals https://venturedeals.techstars.com/pages/coming_soon