When To Pay For Certification?

Some different circumstances around which you may decide to certify a coin:
1. the potential value justifies the fees of certification
2. you wish to sell it with a (kind of) TPG guarantee
3. the coin would really benefit from the protection that a slab gives it.
A combination of two or more of the above.

Strictly speaking, a TPG certification need not take the form of being slabbed. Other forms of certification include:
A. certificate of authenticity from a mint
B. guaranteed auction lot description included on an invoice of sale from an auction house with a good reputation.
C. independent written opinion given by a coin dealer of the highest reputation of international standing.

Thanks for your opinions. I like the "$100 rule".