John Tabita has written several good articles about pricing on Sitepoint, which you would do well to read.
In a nutshell, you have to consider two things – first, what is it worth to you, and second, what is it worth to your client. Those two answers aren't likely to be the same.
First – think about how much money you want to make, what your hourly rate is. If you rate your time at, let's say, £20/hr then work out how much it would cost you to do the work at that rate. Remember that if you are doing freelance work, what you are charging has to cover your tax, holiday, pensions and everything else, so you'll want to charge more than you would get paid as an employee. Don't be greedy, just work out what you think your time is worth.
Second – once you've got your hourly rate and scaled that up to the size of the project, look at the total cost to the client and what they are likely to get back from it, their ROI. If the work is going to cost them £200 and you estimate that it might increase their income by £300, that isn't a great ROI. That doesn't mean you shouldn't offer to do the work, or that you should try to exaggerate the benefit to the client to make it look like a better proposition. Just be honest about what it costs you to do the work, and don't feel you have to do the work at a reduced rate just because it isn't of great benefit to the client – that isn't your problem.
On the other hand, if the benefit to the client is a lot bigger then you can start to look at a different charging model. If you rate the cost of the work at £200 but you estimate that it will increase their income by £20,000 then you would be missing a trick if you didn't pitch for more. If you're going down that route then don't focus on your cost, don't mention your daily rate, what you need to focus on is the cost to the client, and the benefit to the client. If you can demonstrate why them spending £1,000 with you will bring them revenue of £20,000 then that shows a much better understanding of their business than someone who rocks up saying "I charge £20/hr". Then you can negotiate. If they think your revenue projections are over-optimistic, or if they have another lead who is charging them much less, that's when you can start to haggle, and as long as you're still getting more than your basic £200 for the job then you've come out ahead.