Unless you already have rocks in your garden or can get them from some where there is no such thing as a cheap rockery.
Follow this link to see how I built various ones in our garden.
http://www.blogigo.co.uk/Palustris

When i built my first rockery i made sure i had drainage and also hid a hose pipe all round the rockery before setting in any thing else, also make sure you add sharp sand to your soil, goodluck,,, fromTeddygirl

oops i forgot to mention make a few holes in the hose pipe and fix a fitting that will go on to your hosepipe,it works well and stops all the soil pouring out when watering on top,