Off the top of my head I can think of at least two sources for meaningful financial incentives that both are in play in the cryptocurrency scene:

1. ICOs
I know most of these are motivated by greed and the easier execution compared to traditional security offerings. However, they could theoretically
be used to fund development of projects. This is already happening with some Ethereum related projects that intend to offer an ICO to pay
for future development.

The most prominent example probably being Raiden:
https://www.coindesk.com/raiden-ico-ethereum-scaling-solution-launch-publicly-traded-token/

2. Incentive due to own coin holdings
Many Bitcoin or altcoin developers are sitting on a huge stash of their respective cryptocurrency of choice. This means that they are directly incentivized
to improve the protocol, because any improvements would likely increase adoption (or at least prevent the fall of adoption or people switching to a more
innovative competitor), which would increase the value of their coins. Let´s say Bitcoin Core developer X has been in Bitcoin for years and has accumulated a
ton of Bitcoins. He has a direct incentive to ensure the security of the Bitcoin code, because such a high percentage of his overall wealth is stored in Bitcoin.

This is basically the same concept that is responsible for Capitalism working so well - or at least better than other systems that have been tried in the history
of mankind. People following their own self-interest ironically creates effects that are beneficial for society as a whole.

Besides, donations are another source of potential financial incentives. However, these are not really meaningful for most projects, because of the low percentage
of people that actually donates to open source projects.