Their Patient Care Trust has the same vulnerabilities as pensions and personal retirement accounts. They have to take enough risk to beat inflation. In the long-term, that is ... risky.
They only have to beat inflation in cryopreservation costs, though, which, like other technologies, is likely to get cheaper over time. The only exception to that, I think, would be property taxes on the facility (which will increase with general inflation), and perhaps wages for any unavoidable human labor.
One of the sticking points for cryonics is how expensive it is. Unfortunately, the estimates on LW (eg. in Normal Cryonics) are likely to be low as they are current costs. This is starting to come to a head for Alcor, with Alcor's low growth rate meaning it faces a rising tide of aging members (hence that emphasis on young cryonicists) and fundamental flaws in its prices; the official word has come down in the latest issue of Cryonics, issue 2011 q4:
Cryopreservation Funding and Inflation: The need for Action; A Discussion Article by the Management and Board of Directors of Alcor
What to do?
Hope the old grandfathered members like Mike Darwin (who predicted this, in the February and March 1988 issues of Cryonics) can afford that.
On a parting note, I read somewhere that CI's low prices have rarely risen. I wonder what their projections look like...