Chun, who won the Women's PGA Championship at Congressional in June despite consecutive weekend rounds of 75, carded six birdies and a solitary bogey in a second round of 66 at Muirfield.
That gave the 27-year-old a halfway total of eight under par and a one-shot lead over South Africa's Ashleigh Buhai and Sweden's Madelene Sagstrom, with seven-time major winner Inbee Park a shot further back.
Sunday's winner will claim £903,000 from the record prize fund, but Chun revealed she is also motivated by a somewhat smaller sum of money on offer.
"Before the start of the tournament my caddie Dean and I talked about the course and had a little bet," Chun explained.
"If I have a bogey-free round he's going to buy me dinner and pay me 100 dollars each day, so before each round it's like setting another goal.
"I think that mindset helped a lot on the course; I had two bogeys over the last two days but I want to keep trying for a bogey-free round."
Buhai looked on course to claim the halfway lead when she made an eagle and four birdies to race to the turn in 30, but after picking up another shot on the 11th the 33-year-old could not make any further gains and dropped her only shot of the day on the last.
That meant Buhai matched the 65 posted earlier in the day by last year's joint runner-up Sagstrom, while overnight leader Hinako Shibuno followed her opening 65 with a 73 to fall four shots off the pace on four under.
Scotland's Louise Duncan also carded a 73 to slip back to two under alongside Ireland's Leona Maguire, with the English trio of Jodi Ewart Shadoff, Charley Hull and Mel Reid all a stroke further back.
Duncan only turned professional a fortnight ago and after missing the cut in the Scottish Open is now guaranteed her first pay cheque, which could be a welcome boost to the 22-year-old's wedding plans.
"If it's a big, big pay cheque, we can just go abroad and get it over with… in the nicest way possible before that comes across bad," Duncan said with a laugh.
Dame Laura Davies suffered an unhappy 42nd appearance in the event with an 81 which included a 10 on the 18th, although the 58-year-old later joked that it could have been worse after being struck by her own ball attempting to escape a bunker on the last.
That used to lead to a penalty until the rule was changed in 2019.