What is pricing model when you want an extra Sandbox from Salesforce? Is it like how you purchase Salesforce production instance, i.e., $/month/user? Or is it a flat rate, i.e, X dollars for 1 Full Copy Sandbox.
You need to contact your AE for this pricing information. We are unable to provide you an accurate response. It really varies by customers. When you do an upgrade/contract renewal the AE might throw in say 2 partial sandboxes or something. This really just varies by the Edition you are on (Professional/Enterprise/Unlimited/etc) and your contract size. Obviously the more you spend the more likely you'll get some extras.
But... in general it is a % of your 'Total Spend'. I believe the ballpark for a Full Copy (list price) is somewhere around like 25% of 'Total Spend'. So, if you are a Professional Edition client with $4000yr in license cost, it would be $1000. If you are a huge Unlimited Client with $400,000yr in license cost, it would be $100,000... so not exactly cheap in some situations!
This link in developer.salesforce.com describes the differences in the sandbox licences. There is a table which tells how many sandboxes and which types are provided with your licences.
Salesforce license model is very complex, much depends on your total number of licenses and features available. As sandboxes are a copy their price is in relation to your production contract.
Better you'll need to contact your official salesforce account executive for any pricing information.
Pricing of salesforce sandbox is not fixed. It depends on your current contract with Salesforce.
Developer Sandbox: It's free. It is an Individual dev environment with a copy of all your metadata
Developer Pro Sandbox: It costs 5% of net spend.
Partial Copy Sandbox: It costs 20% of net spend.
Full Copy Sandbox: It costs 30% of net spend.