You pay 5% of the build cost in maintenance per year (6.25% in the next version), so over 20 (16) years accumulated maintenance costs are equal to initial build cost.
For expensive (for their size) ships, building hangar PDCs may make sense. This makes sense in particular for ships that were considered fast in their prime:
A battlecruiser 3 engine generations out of date may be thoroughly obsolete, but still able to make a meaningful contribution and keep up with the current battle line.
By current standards, its running costs are exorbitant compared to its capability though. Useful to take out of mothball if available and we expected a major battle, but not something we'd like to keep around all the time... without hangar PDCs, we'd have scrapped it long ago and built something else instead.
While totally cost-free mothballing may be a bit much, I like how the Great Old Ones can return to active service when there is a need.