Well i'd think it wouldn't work simply because someone else would offer it for free, and these companies would lose market share , and plus it would lock out new users cause u would need to give them bank or other payment details, there maybe a niche market for an exclusive premium paid search engine though, but if it has to stay mass market i don't think pay per query is the way to go, also the big internet companies are already making reasonable profits, they wouldn't need to do something like this,