Personally, I ignore exit surveys - I'm leaving, why would I bother spending a precious 10 seconds clicking some check boxes? It's ridiculous, it's lazy, and it's selfish, and it's what the vast majority of users are probably thinking!

Donating to Wikipedia is very worthy, though it may not be a cause the users are interested in. I suspect they'd respond better to a donation to charity, possibly one of their choice. However, users are not necessarily looking at financial incentives to give feedback. The incentive to make a difference to a product, to have some influence, to be credited somewhere may be all it takes.

That particular group of users who did not purchase but would accept a little extra free time might be of value to you - they were about to go, yet you persuaded them to stay. Ask them personally (an email?) - treat them like people, not like survey box tickers.

If you have enough users, have you considered (or already have?) trying A/B testing to see what gets these people to decline/accept your service?

Any chance that you could share a link to your startup so that the community can have a look? [Regardless, good luck, and please report back with any news!]