From what I've seen in my limited experience with aircraft sales, the value of avionics is not found in their reliability as much as their utility.
In other words, the issue isn't that they break, it's that they become obsolete. Sometimes the whole system goes the way of the dinosaur (Loran-C, ADF) while other times the equipment simply becomes so un-sexy that nobody wants it (KLN89 GPS).
It's impossible to predict the future, but I'd budget based on calendar time rather than flight hours. Probably do a panel makeover every 15-20 years. Even then, it depends on the type of aircraft...planning upgrades/refurbs for an SR22 is going to be totally different from my Cessna 140.
To complicate matters further, we have no idea what technologies are coming down the pipe in the future to force down prices on today's avionics. Touch screen displays are the hot new thing at the moment. What will be the wave of the future? Voice-command systems? Heads up displays? Nobody knows.
The bottom line is, it's easy to place a value on current avionics but impossible to predict the future.