My two cents:
Questions that ask for nothing more than a cost estimate should be considered off-topic.
It's really hard to answer cost questions generally -- costs vary from country to country, and they can even vary substantially within a country or even within the same city (particularly true of training costs, where frequency of training can be a huge factor).
A few of them (like the three "How much should it cost to get some rating." questions) are going to keep getting asked, so they should probably be edited to be generic and have a canonical answer posted that lists the requirements and some sample calculations with assumed numbers (like pondlife's answer on the first one). They can then be used as duplicate targets for similar questions about the same rating.
(Ideally I think we should build these in layers - a good answer for Private, then Instrument refers back to Private, Commercial refers back to Private and Instrument. ATP refers back to Commercial, Private & Instrument - so someone can follow the whole chain and we don't need to update a bunch of answers if the requirements for a Private certificate changes.)
Questions that ask for cost factors may be on-topic.
Similar to the logic for canonicalizing a few of the "How much should it cost" questions, we can list factors (and give example numbers) - these run the risk of becoming "List of Things" answers, but I think we can avoid that with judicious voting and editing...