ILblog - you are probably aware, but just in case you're not, you're really dealing with two contracts; PBH for the engines & SBH for the avionics etc.
P&W call their
PBH "Eagle service program", and it operates a little differently to the Turbomeca PBH contract. You "own" your P&W engines, so that if one goes away for attention, you get a loaned spare until yours comes back, whereupon you have to change engines again. With TM you just get a replacement engine & therefore one engine change. Advantages & disadvatages of both schemes - just a different way of doing it. (In a 135, an engine change is a fairly quick job - not so for the same engines in a 902, or much else!
)
SBH can be designed for each customer with as many, or as few, items on the list as you're willing to pay for. Obvioulsy, if a black box fails, it'll often be expensive to replace without SBH, and probably an un-planned cost too. On the other hand, if you're only flying 150 hours a year, the cost of SBH is unlikely to be so great, especially for the re-assurance of relatively quick parts supply under a SBH contract and the knowledge that you won't have an un-expected bill for that box!
A lot of high hour (1000-1500) public service operations in UK have PBH and SBH contracts for two reasons:
1. They can budget more accurately for the hours they're planning to fly.
2. If their aircraft needs a (PBH/SBH) part, they'll be supplied as top priority & will be operational again as soon as possible.
Some units put the equivalent money aside instead - if they're lucky, they'll save plenty of it; a run of bad luck might have them more out of pocket.
At the end of the day, it's an insurance policy to provide (some) peace of mind, which costs, of course - as with most insurance policies, the financial winner is more often the service provider!!
Re CDS/CPDS - I know of one operator who investigated retro-fitting CPDS into their older CDS T1 and the sums made no sense whatsoever.
Buying a CPDS replacement will have worked out a lot better value in the long term.