The cheapest I have ever seen is £2.99 and that is for 10. Obviously you are not going to get top quality for that price, but I bought a set for a fiver for something other than CNC and they work just fine (light duty) - smooth as silk and no play.

I haven't tried skate carriages, but I go on what people write and look for majority views and counter views. On the US CNCZone there are dozens of builds using this method and many look really good for a cheap build. I am quite surprised no-one here has chirped up to say they have done it. Also, the original buildyourowncnc uses this method, but they have changed to V-Groove bearings. Those videos might be of help to you. Some bits look a bit crude, but there is also some good stuff.