What happened initially was I got a lot of interest, and little happened. Now, this could be because I was useless at selling, but I've got a list of everyone who e-mailed me and almost all of them had a domain name, and wanted a website. Not 1 of them has had their website designed, and my main advertising was in August.
At the same time, I stuck on Adwords, 30 minutes later someone contacted me, turned it off thereafter and got a £7k contract from a 93p investment.
I spent so much time altering adverts, measuring etc etc that I have a 20% contact rate on some adverts, and they don't have obscure words in, but receive several hundred queries a day.
It definitely pays off, but it's worth hiring someone to do it for you, as the time you invest into learning it all, optimising it, measuring it and so on isn't worth it, unless, like me, you work in the industry and can advise clients on it.
As a final point, and perhaps it's because I don't have a price on my website, but I do find an awful lot of time wasters, or those just "looking", with the intension of wanting something a few months later, which means they'll use Google again in a few months and choose another design company to take care of it for them.
Oh, and for your actual question: about £300 a month. My average sale is over £2k, so it's not hard to see how it helps me. If you have a good website, good copy and a good advert, you will make a huge return.