Where do you live?
What I usually do is figure out what a ft salary+benefits is, double that, then divide it by 2000 (number or hours in a working year.) So If typical devs earn about $200k per year (this includes salary + benefits + bonus) then I divide that by 2000 = $100/hr
Although people who write checks might use different approaches....
It is generally better if you can emphasize how much you can save the customer (or generate additional income) - and if you can show them that hiring you is a way for them to save money (or generate value) ...
on the other hand companies always cut on you and you can do nothing about that unless you live in US / UK.