If you are on a tight budget, and have an Android tablet, you can get an app called DSLR Controller which is £8, and will act as an external monitor, I have a Nexus 7 and works perfectly. You'll need a USB OTG cable (about £10) and an attachment to hold the nexus, but these are readily available and a big saving on a dedicated monitor. Black Magic have lso just released what looks like a well-priced solution.