OK, I see how to buy them all in one turn
You get precisely $64 and 8 buys. Wow, that's cool!
Have a hand of five Ventures.
Play a Venture1 from your hand, draw and play venture, draw and play venture, ... , and so on until 6 Ventures are in play and then draw and play Counterfeit.
Play Venture2 from your hand, drawing and playing Contraband and then Counterfeit.
Play Venture3 from your hand, drawing and playing Contraband and then Counterfeit.
Play Venture4 from your hand, drawing and playing Contraband and then Counterfeit.
Play Venture5 from your hand, drawing and playing two Banks.
None of these treasures are trashed until after the second Bank is played by this ordering.
$14 from Venture
$4 from Counterfeit
$9 from Contraband
$18 from Bank1
$19 from Bank2
$64 total.
1 base buy, +4 buys from Counterfeit, +3 buys from Contraband.
By the way, it's not possible using treasure alone to get Horn of Plenty up to $8.
Neat. I have no interest in finding the quickest way to build up to this turn, however.