Costs for trials can be controlled if the NRL & clubs are sensible with their selections. Generally the biggest cost for a trial are associated with broadcasting (transportation, lighting and any scaffolding needed for camera angles)
Most broadcast equipment is located in the major capital cities, and generally rule of thumb is if the venue is within a 3hr radius the broadcasters can avoid having to house crew overnight/etc which can blow out costs further.
Generally the broadcasters will absorb a handful of further away games each year, with this year being Mudgee, Mackay and Napier and last being Mudgee & Cairns, but there's plenty of places closer to the capital cities clubs could look at.
IMO even going to places like Bowral, Wyong, Ipswich, Nelson Bay (all places not too far away from an NRL team) is still better than just keeping the games at regular NRL grounds. Does more for the community and looks better on TV.