Of course you could use artificial rock instead of using live but as "aquascaper" mentioned live rock is the core ingrediant to a healthy marine tank.
live rock provides so many things to a marine system upon first set-up which artificial just cannot do.
however if you choose to keep artificial rock, then you will need to do much more frequant w/c
as this rock wont filter out impurities
but corals and things will still live and grow on them, without a problem.
put it this way, being as we are all from the uk the prices on live rock differs.
best live rock by far is fiji.
but in most aquatic shops, your looking at around £12-£14 to the kilo. which all adds up very fast.
but...fear not! i purchased all mine from a website that deals in only the very best fiji rock so good, its full of life before going into your tank.
and there much much cheaper.
there based in wales and there website is
www.live-rock.co.uk awsome rock, awsome prices!
i havent found anywhere that beats it!
and this will still work out cheaper than buying loads of fake rock.
they also, seem to regularly send more than you asked for....i once bought and payed for 50 kilo..and received 70kilo
mix up or not i wasnt complaining.
ive had emerald crabs, bristle starfish, snails, hermits, sea slugs all sorts living in the rock upon arrival.
even small corals and sponges!
check em out before going with fake rock.
best of luck