The lowest perplexity that has been published on the Brown Corpus (1 million words of American English of varying topics and genres) as of 1992 is indeed about 247 per word, corresponding to a cross-entropy of log2247 = 7.95 bits per word or 1.75 bits per letter [1] using a trigram model. It is often possible to achieve lower perplexity on more specialized corpora, as they are more predictable.