Well, I was having trouble getting the achievement, so I turned to Google Sheets (haha). Here's how to get the achievement easily:
Do 1 reroll per round starting from round 3.
Sell a card in round 8 and again in round 9.
Done! In round 10 (the shop round), you’ll have 29 gold.

Round ----Rerolls--------- Sell?
1 ------------- 0 ------------- No
2 ------------- 0 ------------- No
3 ------------- 1 ------------- No
4 ------------- 1 ------------- No
5 ------------- 1 ------------- No
6 ------------- 1 ------------- No
7 ------------- 1 ------------- No
8 ------------- 1 ------------- Yes
9 ------------- 1 ------------- Yes
10 ------------- Store Round

That way:

Gold from rounds: 81 (9x9)

Gold from selling cards: 18 (9x2)

Gold spent on rerolls: 70 (10x7)
Gold at the end: 29