Thinking through the logic, you'd want to create a variable (call it vHours) and create a slider to manage this variable. You can either enter logical minimum/maximum values for the slider, or create a range based on your data...something like Sum([Total Hours Worked]) * X where X is your stretch factor.
I assume you'd want a "cost per hour", so create an expression either in a text box (if this is aggregated data) or as an expression in a straight table chart (where project or person is your dimension for example) that is [total cost]/[total hours worked].
If you then multiply this by your variable you'll see the new "total" cost change depending on the number of hours worked. You can then match it to whatever budgets you have.