The formula we're most comfortable with in terms of accuracy is to divide by 12 and multiply by .85 with earned coins.
For example, (10,000 earned coins / 12) * .85 = 708.33 ($7.08 USD)
With purchased coins the formula is replace .85 with .55.
For example, (10,000 earned coins / 12) * .55 = 458.33 ($4.58 USD)
What is our basis for these calculations? First off the 12 comes from the Tapas store where $100.00 USD purchases 120,000 coins on the app. We also had creators we donated over 120,000 coins to back in December report back what their dashboards said they were receiving (before the dashboard was changed to stop showing individual tips due to the load on the servers).
The .85 comes from the known 15% fee that Tapas charges all tips in order to recover the costs associated with developing, implementing, and ongoing support of the Tipping and Watch-to-Earn programs.
The .55 comes from the summation of the known 15% Tapas fee and the 30% Google / Apple fee structure that exists for any app that uses their payment platform, regardless of who you are. Even Pokemon GO has to pay this. The 30% fee is why you can find Google / Apple prepaid cards sold for less than face value at Best Buy, Costco, Cardpool, etc. at up to 18% off because there's still a 12% margin in it for the retailer and for Google / Apple.
We're not artists. We have to hire out all our art. Everyone does art at a different speed but based on a number of conversations with artists, the median time to do one page of a color comic is 10 hours. That works out to $1.41 an hour, if you should win. If you don't, then your return is $0. Contests are always a proposition. You risk/gamble $X or X hours for a Y% chance at $Z value prize. Given the relative size of Z to X, you're likely to not have a lot of entries.