Estimating the monthly cost of Key Vault

I have created a proof-of-concept app using Azure Key Vault, in my Azure Portal using my Visual Studio Subscription benefits. With that I've convinced my management that we should use Azure Key Vault for storing secrets, like database connection strings, etc.

Now they want me to give them an estimate of the cost of Azure Key Vault. I've found the billing page listing the cost of Standard and Premium Key Vault services. That's nice, but I really still don't have an idea what it will cost. And management isn't content with just that page.

I realize that an answer I'm likely to get is, "It depends". Well, OF COURSE it depends, but it's that which I don't have a feel for. Let me illustrate. I know that web sites are, by nature, stateless. So, every page that needs to read something from a database is likely to open a new connection to perform the read, or any other CRUD operation. And I can see that component technologies like Blazor or React may open the database multiple times when rendering the page. So, is there a way of coming up with a ballpark estimate, based upon the size of the website, number of connections a page is likely to require, relative usage of the website, etc., that will give me a range of invocations of the Key Vault so I can suggest a range of cost per month? My experience with websites has all been on-prem, so I've never considered how often any component is called to retrieve the database connection string from appSettings.json.