Thank you for bringing this inquiry to meta - I've reviewed your question and I believe that it was closed on the grounds that it does not meet the "practical, answerable" criteria from the FAQ:
You should only ask practical, answerable questions based on actual
problems that you face. Chatty, open-ended questions diminish the
usefulness of our site and push other questions off the front page.
Your questions should be reasonably scoped. If you can imagine an
entire book that answers your question, you’re asking too much.
If your motivation for asking the question is "I would like to
participate in a discussion about ______", then you should not be
However, if your motivation is "I would like others to
explain ______ to me", then you are probably OK.
(Discussions are of course welcome in our real time web chat.)
Determining how PHP is using memory requires a great deal of information - even if this information were provided up-front, responses to your question could be expected to vary widely between respondents (the topics of OS, webserver, and database performance tuning could all apply here) - e.g. "are you caching output?" / "do you have X plugin installed?" / "have you enabled or disabled X webserver feature?"
Whether or not a configuration is reasonable is subjective - given that you could easily see many "Dude, you need to do X" replies (as well as numerous requests for more information) this question seems too open-ended to be definitively answered in its present form - there are many potential configurations (and resulting memory requirements).
In the interest of providing resources which will help you determine the optimal configuration for your WordPress installations, I would recommend the following:
The Asking Better Questions article at the StackExchange Blog includes links to a number of suggestions which explain how to get the most out of asking questions - and you're always welcome to inquire at meta if you're unsure of whether a question meets a site's guidelines.