Most of the ring finder services work on a ”Reward Basis”, you pay what the item is worth to you and what you can afford to pay find your lost item. Most have a small call out fee to start the detecting, to cover their fuel etc. Call to discuss.
There are several ring finder services around Australia:
https://ringrescues.com.au/ work in Perth
John Hunt’s Ring Finder service in Melbourne. No find, no fee. Look on Facebook.