The Heathrow Hub plan involves stretching their airports northern runway to 6.8km, and splitting it in half so that planes can take off and land at the same time.
Backers include former British Airways Concorde pilot Jock Lowe, and the group had previously claimed work could be completed for £9.7bn.
However, last week Heathrow offered to cut £3bn from its projected costs of £16.8bn for a third runway, through cheaper terminal buildings and opting for a cheaper passenger transit system.
And now, the Heathrow Hub campaigners have applied similar cuts to their own plans, dropping the bill from £9.7bn to £7.7bn.
Lowe told City A.M.: “If we apply the same changes that Heathrow put on their own scheme, like a lower quality expansion, it saves us about £2bn.
“Personally, I would still prefer to have all the passenger convenience that came with the first plans, but even without them, our plans remain a lot cheaper.”
The government is expected to make a decision on airport expansion after parliament returns from recess in October.
Last year, a report by Sir Howard Davies backed a third runway at the airport over both the extension plan, and expansion at Gatwick.