Various dictionaries have different things to say.
What price [fame/success/victory etc.]?
something that you say which means it is possible that the fame, success etc. that has been achieved was not worth all the suffering it has caused
What price victory when so many people have died to make it possible?
(Cambridge Idioms Dictionary, 2nd ed.)
price [...]
what price (something)? what are the chances of (something) happening now?
(Collins English Dictionary – Complete and Unabridged)
What price something?
What is the value of something?; What good is something? (Said when the value of the thing referred to is being diminished or ignored.)
Jane's best friend told us all about Jane's personal problems. What price friendship? Jack simply declared himself president of the political society. What price democracy?
(McGraw-Hill Dictionary of American Idioms and Phrasal Verbs)
what price ——? used to ask what has become of something or to suggest that something has or would become worthless : what price justice if he were allowed to go free?
(New Oxford American Dictionary 2nd edition, from OS X)
Apart from Collins's strange definition, it seems that the general meaning of "what price X?" is "what's the value of X?" (not "what's the price of X", in the modern sense of price).
As for how it came to be, my wild speculation of the day is that it could have been used grammatically in an anaphoric expression, eg What price is freedom to us if we tolerate this tyranny? What price justice? etc.
This is slang from the 19th century that is no longer used much anymore. Mark Twain probably heard these expressions frequently, as I lay that they were in more common use back then. Some of them are going to stump native English speakers today, as well. I believe I have only seen "truck" used in that sense in books.
For "lay", "I lay" = "I bet". My guess is it comes from "lay a bet on".
Best Answer
Even as a native speaker of a language which doesn't have a similar idiom (German), I always found this one immediately clear and obvious:
Search me! clearly started as a particularly drastic way of asserting that you don't have something. As we all love drastic expressions and tend to overuse them, it was overused and gradually felt less and less drastic.
[Added after Oldbag's comment:]
Equal stress on both words (as the dictionaries say), or even stress only on the second word, makes sense as follows: "Search me for information, and you won't find any!" "Search me for information? What are you thinking?" This is only a variation of the above. It's slightly more complicated, so while the variant that I described with stress on search may exist only in my head, it would also be plausible as an earlier stage that made place for a new, even more drastic way of saying it.
PS: In the specific case that I found described in Webster's, i.e. in response to a question, there is a straightforward interpretation of search me with any stress pattern as short for: "It won't do you any good to search me!"