One way to estimate the cost to write a novel is to break it down. Consider the time aspect. Calculate how long it might take you from the initial idea to the final draft. If it's a year and you work 20 hours a week on average and value your time at $15 per hour, that's a significant cost just for your time. Then there are other potential costs. For example, if you need to travel for research and that costs you $500, and you also buy some writing guides for $200. All these added together will give you an estimate of the cost to write a novel.
To estimate the cost to write a novel, first think about the time you'll dedicate. Say you think it'll take 500 hours and you value your time at $10 per hour, that's $5000 just for your time. Then consider any costs for research, like buying books which might be $100. And if you use a writing app that costs $50 a year, factor that in too. So in total it could be around $5150.
You can start by estimating how many hours you'll spend writing. Then assign a value to your time per hour. Add any costs for materials like notebooks or pens. If you plan to use paid software, include that too.