I'm not sure where you are.
In the UK, it seems to work something like this.
Average cost new is about £5000.
They seem to depreciate by about £1200 in the fist year, with about £800 per year there after.
When they get down to around £1200, they seem to stay there unless they have any problems.
I've seen 6 year old spa's sell for £1200 - £1500 for a good one and as little at £700 for one which requires some work.
Ebay is a great place to use for price comparison.