Assuming you're interested for valuation reasons, rather than US tax
ones...

I did some sums on the depreciation in real terms of an ASW20 over25
years, and got the answer of about 2.3% per annum.

This was based on finding an original 1984 price of $28,000 for an
ASW20C, and approximate asking prices in 2009 of $33,000, then
depreciating according to the retail price index (US, I think, though
I don't recall where I got that from).

Could do with some more data points though.

Paul