I'm gonna try once more using an item with a smaller range, but will leave it there until more feedback is received for this idea.

https://steamcommunity.com/market/listings/440/Strange%20Tipped%20Lid

Keys selling at ~£1.85

Sales:

£2.00 (seller received £1.70)

£2.03

£2.09

£2.12

£2.17 (seller received £1.84)

£4.56 (outlier)

Sellers:

£2.07 x2

£2.12

£2.15

High end = Highest seller + fees (highest someone willing to pay):

£2.17 / 1.85 = 1.17, rounding to 1.2.

Low end = Lowest seller - fees (lowest someone willing to sell):

£2.00 * 0.85 = £1.70

£1.70 / 1.85 = 0.92, rounding to 0.9.

Taking 0.9 - 1.2 as range. Feedback below.