Determining the value of capital assets is partially science and partially art.
I would start by finding out who the local property tax assessor is and then finding out the procedure to request the assessed value in 1990. I know that tax assessments and actual fair market value are often significantly different, but at least it gives us a number that was good enough for the government. (In the US, I’d start with Zillow.com, but even that doesn’t go back 20 years).
There is often a valuation done as part of probate proceedings, so I’d see if there are any documents remaining from that.
I might call some real estate experts who’ve been around that area for a while and ask them if they have anything to go on. (Again, for the US at least, Zillow.com can provide some pretty good numbers for more recent sales).
If all else fails, you can usually find some statistic about the average home value increases over a period of time. Often, these are ridiculously generalized and cover whole cities, states or countries. But they’re something. Work that backwards from the current value and you’ve got an estimate for the old one. This is nice because it (in an indirect way) includes the value of improvements made to the property.
Under audit, the IRS isn’t going to have any numbers that are better than yours. They would prefer some clear-cut document with provable numbers. However, my understanding (which is not from personal experience) is that they’ll accept a reasonable method used to come up with a valuation for the property. So you document the research and any opinions you got and run with it.
(And if you think a 20-year-old condo is bad… just try finding out the value of a farm inherited 50 years ago. Or stock that was purchased before it was publicly traded.)