I'm planning on picking up the nexus 5 which has just come out today, but it's £300.
You could try picking up a nexus 4 on the cheap, google were selling them for £169 new last month so there should be a few on eBay.
P.s. I've got an ovivo sim which gives free minutes, text and data each month on pay as you go, so I shouldnt need to spend anything per month. The downside is it redirects you to an advert page before you can use the browser.