You can achieve this by passing the values to the standard field id’s in the url. In this instance, you’d retrieve the standard id of the “Amount” field, and then use this Id in the url to assign a static value to the Amount field. Override the standard “New Opportunity” button to redirect to this url, and you are set to go.
You can prepopulate fields on tha standard layout using Salesforce URL hacking. In this, you have to pass the values to the standard field id’s in the url.
NOTE: Since the Id is different from in production from Sandbox, so you need to store the value in custom setting.