I've read somewhere, and I believe it's true, that if someone will have to choose between two designers: one, who designs a website for $500, and has no results, and the other one, who designs a website for $3000, and can prove 40% increase of sales, will choose the second one. Other words, the best way to advertise your design services is to say something like that: "With a website designed by me, your sales will increase by XX%". As I said I believe it's true, but how does one achieve that? Do you have any method to measure this?
The only thing you can prove in design is that the design will look nice and be effective. Promising anything else (especially percentages and numbers) is a disaster waiting to happen!
What do you mean saying "be effective"? Be accessible, usable and standards compliant? Something more? Anyway, I'd like to enhance my campaigns, and I started thinking in this direction, but I don't want to promise anything more that I can deliver...
Well, it actually depends .. Somebody may be able to do a great job even with a low pay. It isnt the matter of how much you pay. Instead its the quality of the work you receive.
effective in this sense means that you've designed it to be easy for the average web user to follow - everything is optimized correctly and works fine. Beyond that, there is nothing else you can promise.