My best buys were mostly all 6 or more years ago, before the cast iron renaissance of the covid era. I think I'm approaching 10 years in CI collecting. It takes time and patience to find the good old stuff. Though on average my $5-$10 deals have moved to $15-$30 deals. Cast iron hunting is not immune to inflation either. There are more cast iron pickers every year and they aren't making any more of the vintage stuff, so supply is fixed.
Part of the country you are in greatly changes your availability of good deals. Here in TX away from any major foundries means hunting far and wide for the real collector grade stuff. In the north eastern US, I see posts of $8 Griswold LBL's at the Goodwill and my jaw hits the floor. They are out there, you just have to get lucky and hit up the places cast iron is likely to pop up. I like flea markets and antique malls personally.
$5 BSR red mountain #9 skillet. Flea market
$7 BSR red mountain #7 skillet. Flea market
$8 Wagner unmarked chicken fryer. Flea market
$25 Griswold chicken fryer with lid. Antique mall 8+ years ago.
$25 Wagner 1060 #10 skillet chrome plated with heat ring. Bought from the same booth at the Antique mall with the above Griswold. Both for $50!!
$30 Lodge 1-notch hammered #9 skillet with lid! Antique shop. This was just last year! Has severe interior pitting though.
$40 Erie 3rd series #8 skillet chrome plated. Canton Flea Market. I didn't have a clue how rare of a piece this was when I bought it. Used it for a year before I started using other skillets and put this in reserve.