The best thing to do is to make your own, it will knock the spots off most things commercially available for the same price and in the process you'll get a better understanding of how they work and therefore (I can only guess as I'm just starting to build mine) will be better equipped to fix them when things go wrong/break.

If you look at, for example, the MD ones and the cheap China ones and then look at the build threads you will see the difference between a "home made" one and a "commercial" one. Making your own will enable you to do things like use supported rails, make the gantry much stronger (and normally a bit lower) use better quality materials for the same price you would have paid for a ready made one. There are enough guys on the forum here who will cast a critical eye over your design and once it's complete do any machining for you if you need it allowing you to assemble it with simple hand tools/workshop tools.

In my opinion a ready bought machine is only really good for scratching the surface at playing with CNC machines, in more ways than one! You will undoubtedly, no almost certainly from my reading various forums, need to update components to get the performance and finish you require. I did consider going down the ready made route before I started but soon dismissed it and now I will end up with a machine that will be capable of cutting Aluminium even though I started out wanting one that would cut Balsa and ply.

As an example of costs the MD A4 ball screw machine starts at £3,555.00 For that you'll get a machine that has a machining Area of 300mm x 220mm x Z Axis 155mm, my machine (when I get my finger out and start making it) should come in well below that and will have a cutting area of 1000 x 500 x 200. As far as I'm concerned it's a no brainer! If size is an issue, i.e. you don't need a machine that big then you'll be able to make a much stronger/accurate machine for half that cost.