I just picked up a bottle of Kirland Signature Blended Scotch at Costco to make rusty nails after viewing this thread one to many times. I haven't had Drambuie, or a rusty nail in several years, but I made one (just scotch, Drambuie, and ice) a couple of nights ago and it was delicous. Much better quality than I expected, but YMMV.
They have two KS blended scotches, both 1.75L bottles. The regular is $20 here in KY, and that is the one I purchased, but they also had a 12-year old blended scotch for $40.