I'm not so sure ... if someone came to me asking a complete build and test, it would take me a while to come up with a figure that I'm happy with.
Look at it this way (skip to the bottom for TL;DR if you like!)...
There are two factors to consider for the kind of freelance work we're talking about:
1) How long will it take?
2) What is the value of the work?
The first of those is a function of your experience and the requirements of the job. For example, you specialise in email marketing, so you will be able to produce an accurate estimate for designing, building, testing and launching a given email specification. Complexity shouldn't really be an issue; while your day to day job might involve creating large email campaigns for distribution to millions of customers using many different email clients, you should still be able to estimate for a single email, distributed internally only, with all recipients using Outlook.
(If of course you are being asked to do a job that's outside your specialist area - for example building a webpage when you've never worked with HTML and CSS before, or only very briefly - then you will have a problem generating an accurate estimate. That's when you need to consider whether you should be taking the job on at all.)
Calculating the value of the work is harder, you need to consider three things:
- You: what is your time worth?
- Client: relatively speaking, what is the work "worth" to them?
- Market rates: what would others charge for the same job?
Everyone should be able to answer the first of those. Start from the hourly rate of your day job (that's a base figure for what your time is "worth" to your employer), double it, and go from there. Consider how much you need to get paid per hour to justify not doing something else with your personal time eg. hobbies, seeing friends, taking the kids out?
The second is relatively straightforward to work out if you understand the meaning of what you're doing (that is, if you appreciate why email marketing is important, rather than just the technical aspect of making sure an email works in different clients). Of course, a lead developer (or email marketing consultant) should always know why their work is important! You'll need to consider whether the client is small or large, the customer base they're targeting, whether this is the first webpage/email they've ever had, the expected return on the work and so on. Basically - will they pay a premium to get somebody good, make sure the work is done properly and have an attractive design which will drive new sales... or are they a fledgling business sticking a webpage up online with an address and phone number on it as a placeholder, doing it on the cheap, and don't really care what it looks like as long as it's tidy?
Finally, market rates - the most difficult obviously as it will vary so much. Do you live somewhere with hundreds of unemployed "web designers" lining up to take on a quick bit of portfolio work for pocket money, or are you in an agency-rich area where talent is snapped up and sold on by a bigger brand? Although I can see this being the easiest piece of knowledge to have lost as you advance in the ranks of a big company, it's also not particularly difficult to research. Again, looking at your salary and your job role at your current place of employment will be a good start on this one.
Wow, this post got a lot longer than I intended...
TL;DR: Although it might take you "a while" to come up with an amount that you'd be happy charging, I think anyone with experience in a field will still be able to work out a figure that's appropriate for both themselves and their client. BUT lack of skills or experience in the field will (as you'd expect) make it much harder.
