If you were making a logo/header for a website, how do you know how much it's worth? Is there any way to get a quick and free evaluation for it, or do you learn it as a result of experience.

depends on if it's for someone you know or what the ciricumstances.... also depends on what they want if it's something that is only going to take you 10 minutes to do you obviously wouldn't charge as much as you would for something that was going to take hours to come up with. Something else I would consider is how much information they give you because that is a factor in how long it will take for example if they said "I would like a banner with some sports stuff on it and make it fit on my site at www.mysite.com " it leaves alot of work for the designer to figure out what to put on the banner but if they said "I would like a banner 800px x 200px and I want a blue background that fades to white on the left side of it with a basketball, baseball, and soccerball on the right and the corrisponding goals on the left" that requires no time trying to come up with something yourself they've told you exactly what they want and that could be done in under an hour.

True, thanks for the info. I was thinking the same, but I wanted someone else's insight. The point you made, about the customer telling you exactly what they want is very true. It speeds up the process a lot, and it avoids conflicts, since you have an idea of what the customer expects.