I can't understand why your old place isn't still worth $170K - is the area or the house different in some way these days to justify more money?
As far as I can tell, it's still the same - maybe painted again.
I'm sure the value of any house is a combo of buyer demand, plus the construction costs if it were to be replaced.
For example, I think from memory it was a 15 square house, and if it costs around $11k per sq to build these days (for a basic product) then this makes the house alone around $165k, plus the land (it was in Boronia, so not a real flash area).
Based on that, the value would be a bit low if you allow around $150k for the land.
At the end of the day, there's just no real hard and fast formula.