There has been an increase in piston twin prices over the past 1-2 years overall, but mostly for twins at the top end of the market. I recently saw a very nice 340A sell for around the $500k mark. There's no way it would've sold for that a year or two ago. Even today it surprised me a bit. I'm seeing top end 414As and 421Cs also go for around that half mil mark as well. Earlier 340s, 414s, and 421s I've seen tick upwards a bit but still represent a lot of plane for the money. These planes represent something that is just not available on the market in a new aircraft. Some people are picking these instead of turbines because they make a better mission fit. There have been a few interesting stories of people "downgrading" to a pressurized Twin Cessna from a turbine and find they like the plane better due to it being a better fit for their flying.
The older twins tend to not get as much of a price boost, although they do get a slight residual increase. Call it trickle-down. The people at the top end of the market are driving up the prices on the newer planes, which drives some people to pick up an older plane since that's what is now in their budget, driving those prices up. But it still only works for a few years back and then prices more or less just tank.
While I realize many of us don't like a price increase, there are benefits. For one, it means more airplanes are flying. For two, a higher value means that more airplanes will be worth making repairs and upgrades required to keep them flying, rather than send them to the salvage yards. Many planes are going there now because the planes are worth more as parts than as planes.
