I'm gonna tell you a story. You go to a party and you walk up to the guys and you're like, "I just bought a two thousand dollar big screen TV, and surround sound." Or you walk up to the wife and you're like, "What do you think of my new fifteen hundred dollar earrings I bought for you." And nobody bats an eye. But you mention once that you're thinking about spending seventeen hundred dollars on a set of headphones, and everybody loses their mind! And I don't understand why.
Took me a sec to find everything. Style guides differ, but AP Style (the authority for journalists and in corporate communications) uses numerals for numbers greater than ten. Nine, ten, 11. Nobody will criticize it, since it's understood we use different guides. AP Style is the easiest, and you're shortening numbers considerably when you use numerals. There's no style guide that recommends writing out numbers larger than 100.