How to save textbox value in the database

Question

How to save textbox value in the database (Textbox value populated based on other textbox input value)

Answer 1

Solutions proposed in comments to the question seem to be reasonable, but I would offer one more if you don't mind. If TotalRate field's value is somehow important/vulnerable then I would never store it in database to avoid possible malicious manipulations from app users. In case there is some good reason/requirement to store calculated value in database I would go with calculation on the server side and handle it in onBeforeCreate and onBeforeSave model events:

// onBeforeCreate/onBeforeSave event handlers
record.TotalRate = record.PerHourRate * record.HoursNumber;