Not very rare, at least 20% of the date are Indians and according to the article 30% of the Indians are flat based. So roughly 6% of the 1922s are of this variety. The 1922 penny is about the most common Geo V penny to find (along with 1927 and 1936) in bulk lots so with a few thousand 1922s I've picked up scores of this variety (all average circulated) at under 20 cents each. They only get hard to get (like most older pennies) as you move up the grades (thus the article calls their 1922 in MS64 "extremely rare"; but not worth the price quoted.