Doing a little digging for my annotations: HPL’s “The Nameless City” was published in 1921, and Howard Carter had not yet found KIng Tut’s tomb, so we can’t blame Egyptomaina for the story (which I quite like, incidentally).
The most likely source of inspiration is The Thousand and One (Arabian) Nights. Checking the contents of the English translations, we find a short story about Irem, a long-deserted wealthy metropolis smote by God (shades of The Doom That Came to Sarnath?) and a more obscure story called “The City of Brass” which contains (among other elements) an ‘archaeological’ expedition across the Sahara to find an ancient lost city, a mummified queen and petrified inhabitants.
I present them here, for the interested reader:
“…and there came to the builders’ hands of all these things so great a quantity as may neither be told nor imagined.”
–Payne’s translation, The City of Irem
Earlier, the narrator counted one thousand trillion (people) (using 100K x 100K x 100K), so this number must have been pretty high indeed :)