You have to see the whole picture. There is always more being transacted than simply dollars for lines of code. When you grok that, you see that "salary" does not make sense as the "free variable" in your valuation equation.
Legal commitment isn't the only kind of commitment.
In consulting/contracting/freelancing terms, if a client accepts a multi-year retainer with a large number of hours, you'll likely give them a bit of a discount relative to your normal everyday rate. They're committing to continuing your business relationship for the duration of the contract, even though the contract will almost always state that either side can end it at any time with a certain amount of notice (just like full time salaried employment).
eg for bog standard perl work in UK Central London) id be charging a minimum of $700-800 perday.
Minimum Rate = (Potential Salary * 1.25 – Freedom Tax) / 1000
When negotiating with customers, the formula will be:
Offer Rate = (Potential Salary * 1.25) / 1000
Potential Salary is the area where your value is estimated. Since the customer will likely negotiate down, the minimum rate is simply the point that one should never go below.