For example, how long does it take to install / configure Apache Solr as the search engine for a Magento CE build?
Perhaps a related question (or a part of that question) would be to break the task down into the different sub-tasks involved.
I wanted to reply in comments but had too much to say, so here are some responses to some of the great feedback I've received thus far.
Precedent in other SE sites
It looks like cost estimates are not allowed on Stackoverflow, but are allowed on Programmers. They talk mostly about the reasons for not asking about costs, but they don't mention things related to time estimates specifically. Also, part of the reason I thought to ask this on this site is I get the sense this will be a more business-friendly site than others given the nature of the Magento community. Not sure what that means exactly, but trying to feel that out.
Depends on experience
While it's true that any answers related to experience do depend highly upon an individual's experience level, that's not necessarily a problem. It's actually very useful for me to know that it's possible for Solr to be deployed in one minute for a highly experienced and specialized individual. It doesn't mean that I would use that estimate myself, but it's useful information none the less.
You might say that multiple people could submit completely different valid answers - one person says it would take a week and another says it would take a minute. Well for starters, that information alone would probably tell you that the person who says it would take a week is significantly less experienced (and perhaps trustworthy on this particular topic) than the other - which could, again, be useful information.
But also, there are many questions that don't always have One Single Correct Answer to them, such as What is the Best Magento Server Setup?.
Estimates tend to lead to good questions
I find that the process of thinking through an estimate can lead to a lot of good questions, such as the ones that you posted related to Solr installation - sometimes even more so than if someone simply asks a more generic How-To question.
I can see the concern with someone just irresponsibly posting a question up for an estimate and giving that to their client without any further due dilligence or thought. Of course that would be irresponsible, but ultimately it's not our problem to ensure that users are responsible.
I also saw your other question (Should those profiting from answers be allowed to ask questions?) which seems somewhat related. It seems to me that the vast majority of questions on any SE site will be directly or indirectly related to profit and I actually think that's a Good Thing.
You also say that it could promote people carrying out work that they aren't capable of completing. Aren't all users asking questions attempting to get help with carrying out work that they are not currently capable of completing?
And, in fact, this could be a safeguard to end clients, because if irresponsible developers are getting their estimates from reliable, trusted sources on this site (reliable, trusted sources are what SE sites tend to produce and highlight), then it should decrease the chances that they would unrealistically bloat estimates.