Here is a link to a basic model - http://monetizepros.com/tools/template-library/subscription-revenue-model-spreadsheet/
Depending on the purpose of the model you could get much much more elaborate or simpler. This base model will help you to understand size of the prize. But if you want to develop an end to end profitability model (Revenue, Gross Margin, Selling & General Administrative Costs, Taxes) I would suggest working with financial analyst. You biggest drivers (inputs) on a SaaS model will be CAC (Customer Acquisition Cost, Average Selling Price / Monthly Plan Cost, Customer Churn(How many people cancel their plans month to month), & Cost to serve If you can nail down them with solid backup data on your assumption that will make thing a lot simpler.
Let me know if you need any help. I spent 7 years at a Fortune 100 company as a Sr. Financial Analyst.