Ooh, I can play this one! (I'm so cheap that my most expensive BJD was on clearance, hah)
My loudest "cheap" doll is Luke, in that he had no character when I started and has become one of my top 5 OCs.
Thrift store Beach Fun Blaine head (Goodwill, $2), originally on a spare 1999 Ken body but recently moved to a super-articulated Joe body, which was either $3 or $4. (I had a few and the pricing was inconsistent on them.) So... $5 or $6.
Wes is an honorable mention there. Since he was one of my characters to begin with he's one of my favorites.
Head (some HSM Ryan + apoxie) was in a $4 dollar bag of 3 dolls at Value Village, so I'll say $1.30. Body was the first super-articulated Joe I got during the Goodwill X-mas sale, so $1. That makes him $2.30.
Then there's my cheapo BJD-like... $1.30 Liv at Goodwill. The wig was a gift from my sister, a $5 wig from a doll show that didn't fit the doll she bought it for. But to be fair I'll include it in the price. So she's $6.30.