if i'm reading the charts correctly, YLI Soft Touch is the thinnest of the cotton threads listed.
seems to be generally less expensive than aurifil. (not "dissing" aurifil. i love the stuff.)
the lowest price i have found so far for 1,000 yard spools of the YLI is here:
https://www.craftmore.com/sewing-qui...sort=alphadesc
for 6,000 yard spool/cone, this is the lowest i've found so far:
http://www.uncommonthread.com/catego...how=100&page=1
i swore i wasn't going to buy any more thread until i'd used up the huge collection i have already.
i shoulda knowed better 'en that! lol