search engines are getting smarter with typos, some of them now automatically revert to the correct spelling for the search, or if you search for two words together, like 'carparts', it automatically splits up the words for you. this will make it harder in the future to get search rankings for typo domains and two and three word domains.
as for search volumes of typos, if you are seeing the typo being used in lots of webpages, then it's obviously a sign that lots of people are commonly misspelling the word. 250,000+ pages would be a high count IMO. You also have to consider if the correctly spelled word is a trademark, or if the correctly spelled word even gets any type in traffic.
You can also check the search page returns of the typo vs. the correct spelling. Say if you get 250,000 seach pages for the typo and 2,500,000 pages for the correctly spelled word, that's a ratio of about 10%. If you get 50,000 for the typo and 5,000,000 for the correct spelling, that's 1%. good luck!