How much will it cost .. three posts a day and 15 comments?
Post and comments are essentially the same so this is 18 posts+comments and answer to this (given the current state of the RC system; these numbers may change but probably not by a huge amount absent some design changes) is about 10-15 SP. It can be actual SP in the account or delegation.
Except that's not the whole answer, because on top of the basic charge for simply doing the post transaction, there is an additional charge on a per byte basis which increases the overall cost. What is "the target post/comment size?"
We don't know. And just as importantly, we don't know what it will be tomorrow because one of the major witness values is not only what the transaction cost is but what the per byte cost is.
Given the current state of the RC system, these numbers are probably going to change pretty drastically because the current state is definitely not what was intended and what is intended. If nothing else, the 1/10 cost declaration from the witnesses is very likely to slide up over the next month or two. At the very least.
I suspect that you probably can't get along with less than 20 to 25 SP at the current settings and I have no idea how high that's going to go up to in the future, because no one with the power to make that decision has had a conversation of where they want to see it go.
Doubt and instability are not good for encouraging investment, whether it be financial or personal interest.
It is close. My estimates were for a 'moderate' size post or comment of 1-2 KB. For larger or smaller the numbers will vary. The per byte charge is pretty low and dropping, so the effect of size is becoming smaller.
These aren't currently adjustable by witnesses. The rules are fixed (absent a software upgrade), although the prices are determined by market/usage conditions so in that sense you can't know in advance, only estimate. C'est la vie.
The pool formulas have a half life of a week, and they were pre-filled to 90%. As such it won't take that long. There is still some adjustment going on as things balance out but it isn't enormous. When the 1/10 as applied there was an immediate dramatic effect (15 SP accounts were limited to 1-2 comments previously). There will be further adjustment to equilibrium but only around the edges. For now it looks like the most significant change will be a smaller cost per-byte, reducing the importance of post or comment size.
It clearly depends on what you are trying to do. The same numbers won't work for everyone. (Also some differences for different apps.)