This site has some customer reviews of various dating sites and a couple of them include pricing info. What it says for match.com, for instance, is that it's £23 for one month, but if you join for three months (which is what I recall I did), you pay £16 per month.
I know that there are some free sites out there but, in my experience, you get more "serious" people if they're willing to pay to meet others.
The link has the name of the web-site where I met my DH - makefriendsonline.com. Like so many of the others, it's free to join and if you want to upgrade, then you find out what it costs. Neither of us are members anymore, but I think it was around £15 a month. I liked it, I wasn't looking for a date though, I was looking to make friends online - looking for friends for when I moved to the UK, and there are loads more British people on there than any other nationality, although I think the site is based in Canada.
Anyway, I still keep in touch with a couple of people I made friends with there - one guy, one gal, and my DH and I are in very close touch, more than 4 years after meeting there...