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.
I have ample experience valuing SaaS-based business models from my recent operational experience within a SaaS business and my previous experience in investment banking. Happy to provide more information, but a "good" projection model will require a strong (and well thought-out) operational build-up. The more details, the better.
I make advanced financial models. I have one for SaaS and enterprise SaaS. If you are looking for something basic, then there are some free ones available. Chris Janz's is the best.
You may grab one of my cases here: https://finmodelslab.com/simple-saas-financial-model/
Looks like the other experts have links to SaaS financial templates. Once you have a chance to look these over and find one that you like, I can talk you through how to tailor it to your own business model - or how to use a financial model to inform your strategy.
Happy to talk you through this so set-up a quick call if you'd like.