This is the situation that shares were intended to solve.
They're not perfect, they're not ideal, but they do at
least provide a way of thinking about the problem. This
is simplistic and intended to give you a point of view
from which to reason about it. It's not the end of the
story.

Let's suppose there are 100 shares, and you each own 25
of them. You can't think in terms of the effort you've
put in so far. That's sunk money/time and you need not
to think about it at all. You need to think about the
value of the company/idea/project as it is now.

And that's right now, not in two years time. You don't
know what it will be worth in two years time. Given that
you've lost your enthusiasm, in your heart of hearts you
must think it's not going to be worth much in the future.
If you honestly, truly believed it was worth pursuing
then you'd take a day or two off to recharge, and then
get stuck in again.

But you want to hedge against the million-to-one chance
that this will all take off without you. So value your
expectation of the company's worth.

With 50% chance, you think it's worth nothing.

With 30% chance you think in two years it will pay the bills,
but only just. Company worth - $100k

With 15% chance it will be a really good, on-going business.
Company worth - $400k

With 4% chance it will soar and be worth $1m.

With 1% chance it will really take off and be worth $7m.

Is that how you see it? Change the figures to match.

That makes the expected value ofthe company:

0.5*0 + 0.3*100k + 0.15*400k + 0.04*1000k + 0.01*7000
= 0 + 30k + 60k + 40k + 70k
= 200k

So your share at the moment, based on this belief, is currently
worth $50k. Each share is worth $2k, and you have 25.

Tell me - would you sell me your holding for $50k? If you
snapped my hand off, then that valuation is too big. Are
you really sure you wouldn't sell it to me for $50k?

Honestly?

So you need to adjust those figures down. Honestly, how much
would you take for your 25%?

Now you want to walk away and leave your colleagues to take
all the risk. The point is, they will continue to put time,
effort and money into the venture. As they do so, they
should be compensated, but you shouldn't. So they need to
put in X and receive newly minted shares in return. Yes,
your holding gets diluted, but the value of the company
should be going up. The value of your holding doesn't
change, so long as they're getting linear returns for
their further investment.

Suppose they all put in an additional $10k. The company
will then be worth $230k. They should each receive 5 newly
minted shares, so there are 115 shares issued. You now
hold a smaller percentage of a larger valued company. You
have done nothing, and the value of your share holding
reflects that.

Suppose they all put in another 6 months of work. How
much is that worth? Based on what's happened so far it's
a quarter of the time spent so far, so they should each
get an additional 6 shares.

And so on.

Their proportion of the pot is increasing because they
are continuing to put more in.

Now here's the kicker. You can trade your shares with
them. If your valuation of the company is greater than
theirs, you can put money in, getting shares in return
based on the mutually agreed valuation (splitting the
difference). On the other hand, if you think the
company is going down but they don't, you can offer to
sell back you shares.

So here's the question again - what would it take for me
to buy the 25% from you and have you walk away
completely?