I've heard around the £20 mark cited before but I don't really know as I haven't bought from him. There are a couple of decent ebay sellers based in Japan, who will charge anything from £10 to £20 a pop. It's probably more than you'd pay actually buying them from a store in Japan but there's little other option if you're importing to the UK. I usually pay about £7-£8 shipping for airmail, which is steep for a single doujin but works out okay if you're buying a few. Some sellers will offer SAL fairly cheap or even free but it is also desperately slow. In terms website I've heard Mandrake praised but have never used them. There's also Mangamon, who I haven't bought from directly but have used their ebay store. J-list sells a few but from what I've seen more female oriented than yaoi. Cheapest you're going to find is buying second hand on ebay, which I've gotten anywhere as cheap as £5, although then you're limited to what people are willing to sell.
Hope that is somewhat helpful!