4kg is not a lot on a 300L system.
Anyway, check out the for Sale forum Sometimes bargains do come around. But notmally they are rock in a bad state. It is liverock, but either overgrown with hairy algae or briopsis or so loaded with phoshates. But with a bit of waiting and keeping them in the dark - that is what they call cooking your rock - you can restore that rock to what it should be.
But even "good" quality rock from a LFS could have a nasty surprice or two. As these are new rock from the ocean with unwelcome hitchhikers.
But yes, life rock is expensive. Idol Marine in Midrabd s one of the closer LFS shops to Bryanstone as far as I know.