Another option is to buy an on-site van. You will be surprised at how cheap some of these are - several under $10,000. major draw-back? You are stuck at that particular park, but, if the majority of your time is in the Vic goldfields - you are very central to all the major detecting spots, literally at your front door. Ongoing costs can be minimal, in comparison to paying full tote odds to stay at a park, they have OK (not brilliant) re-sale value. Secure, reasonable facilities etc. I realise its still a cost,but you will be amazed how many detectorists either have one or started out with one. Some parks do not allow full time residents so there is at times a limit of nights you can stay - usually about 120+ I think in some cases.
I just spoke to a mate - he pays about $60 per week for fees and electricity at Laanecoorie and paid $8,500 for a van/annex/small shed - nothing flash. You really can't get a better detecting location that close to Melbourne, Bendigo, ballarat, Dunolly, Tarnagulla etc. $8,500 is what, $2,000 more than a GPX5000.
Just a thought, but still big bucks I guess.