Crushed Coral, Aragonite or Beach Sand
The only thing that makes Live Sand 'live' is the bacteria and small organisms living on and in the sand grains which are kept alive by packaging it with natural seawater. This means it costs more to collect, package and transport.......all things that you end up paying for. These same bacteria and organisms are found in Live Rock and once the two are placed together the bacteria and organisms will colonise both so all you are paying extra for is pre-colonised sand. Admittedly Live Sand will speed up the filtration capabilities of your tank but IMO, not enough to justify the difference in price - £25 a bag for Live Sand, £3 a bag for Crushed Coral (same quantities)
IMO you either do cured Live Rock and a 'dead' substrate or Live Sand and uncured Live Rock