There is no fixed sum. But if you are a skilled professional you should calculate "opportunity costs": If you could get a job with 250k payment and you pay yo

There is no fixed sum. But if you are a skilled professional you should calculate "opportunity costs": If you could get a job with 250k payment and you pay yourself only 50k - you are theoretically loosing 200k a year. If the same startup generates 200k of profit for the first year and you own 50% if it you are still loosing 100k. Then again if this startup could be sold right now for 500k - your up 150k. Next step would be probabilities: If you have no offer on the table but there is a 80% chance your startup could be sold for 1m, calculate with 800k. And stop adding this extra value if your exit probability is low.