If you are in a small niche area eg a small town and not many competing it would be very cheap. If you are competing in London or other major cities it will be expensive, especially if using adwords or similar. I will do a simple 2/3 page website for £200 ish, supplied and fitted with your own domain name, if you provide the copy images etc (if you want images).
When I say expensive I mean expensive to compete for keywords, the actual website is the same price no matter if 10,000 see it per day or 10.
The cost is taken for getting found, links etc. That can be left open, and up to you.
However if in a reletive small geographic region and that is only the area you wish to work, no problem, free directory links will get you found
Domain reg is in the region of £20 per year depending if .com or .co.uk, hosting £40 ish per year (depending on how you want to go, it can be much more expensive if you want a shopping cart etc, if you supply goods and take orders etc etc)
I 'm thinking you are basically just looking for an internet presence, contact details what you do etc, if so it would cost in the region of £200 for the first year, including website design. thereafter just the cost of hosting and renewal of domain name, approx 50/60 quid per year..for nothing I would say. (assuming you do not want to alter an existing website substantially.
Most people whom search for services are not interested in artistic impression, an electricians website does not have to be a masterpiece,,,though it would not hurt.
Most potential clients just want an email or phone number, what you do, plain and simple.
You do not need to pay £1000 for some luvvy-dovvey artistic impression.
The web spiders love unique content, difficult I know we all do the same thing in different areas of the country, so your site needs to have better content than your local competitors and better quality links...easy!
If a reletive small niche area, I will get you in google maps and (these will show in searches, for example - electricians blackpool - I've picked that one because it would be relatively easy (assuming your bizz is located in blackpool) Try - manchester electricians - to see what I mean, that would be whole lot harder, but not impossible. Google maps is one option because its free, getting organic results is also free but in competetive areas very difficult unless you are willing to spend yellow pages type money.
But like I said an actual website is very cheap, if you live in a none competetive area it is brilliant and more or less free.
Having said that, even if you live in London, you should have a website just to give credibility.
If using google adwords you need to watch your budget very carefully, and your target keywords, you can easily rack up £50 per day. Target keywords need careful study.
Hope I've given a few pointers