Revenues:

1. List your stakeholders
2. Define a timeframe you want to considers (e.g. next 3-5 years)
3. Define the revenue streams for each stakeholder segment incl. your monetization model
4. Sum up all revenues

Costs:

1. Define the costs for acquiring the stakeholders
2. Define the variable unit costs – e.g. how much each transaction will cost you
3. Define the operational costs – what it costs to run the platform business
4. Sum up all costs