First, we should understand the revenue model. For example, it could be a quantity x price model, subscribers x ARPU model, etc.
If the revenue synergies come from increasing one of these variables in the revenue model, such as an increase in subscribers as a result of cross-selling, then we should estimate how many more subscribers we would get as a result of cross-selling and apply an average price to each category of subscribers. We can analyze the historical data to see how effective cross-selling has been in the past.
If the revenue synergies come from a new revenue source, such as expansion into an adjacent market or new geography, then we should estimate the quantity and price of this new market. This can be done through a top-down method, such as applying a market share % to a total addressable market size. A bottoms-up approach can also be used, such as estimating the # of new customers and the dollar amount of sales each employee in the sales team can generate in a new market.