Folks,
Based entirely on my personal experience, not second hand:
1. The condition of some C-402 of various models has been "scary", not only extensive corrosion and cracking around fastener hole in the inner wing/undercarriage/engine mount area, but elsewhere in the airframe, including serious corrosion up the fin.
In may cases, the "normal" level of inspection at an annual/100h would not have detected it. Don't blame LAMEs, ain't their fault, either the problem, or the cost of repairs. Several orgs. have set up semi production lines, this reduces cost for the 300/400 SID, talk to AMROBA to find out who they are.
Believe me, this is NOT a business development exercise by LAMEs, or Cessna.
2. Less experience with 300 series, but I have the impression that the problem is not so extensive, and the resultant cost of a SID is not so great, but still is a serious amount of change. From owners I have talked to, for a -310, the $40,000+ to $70,000+, around half a 402 cost ---- but these are ballpark figures, and nobody can give you a "firm" price, because you don't know what damage needs to be repaired, until you tear down the structure.
At least in Australia, we have some very good NDT people, including being able to do crack detection in fastener holes, without necessarily having to use the fastener.
3. Strutted singles: Best summarized by saying that, based on corrosion I have personally observed, for the last 10 years or so, I would automatically factor in the cost of rebuilt wings into any purchase, and that has been my advice to others, when requested.
It is a testimony to the soundness of the strutted design, that this problem has not, apparently, resulted in recorded in-flight structural failure. But, then again, many GA accidents are not fully investigated, so I wouldn't rely on the record so far.
Tootle pip!!