Here's the Exact formulas:
Auction Fee/Commission = (0.5 + Markup * 99.5 / (1990 + Markup))
Shop Tax = (Markup * 49.75 / (1990 + Markup))
===
quick reference
Markup - Auction Fee
0 PED - 0.5 PED
5 PED - 0.74 PED
10 PED - 0.99 PED
20 PED - 1.49 PED
30 PED - 1.97 PED
50 PED - 2.93 PED
70 PED - 3.88 PED
100 PED - 5.26 PED
150 PED - 7.47 PED
200 PED - 9.58 PED
300 PED - 13.53 PED
400 PED - 17.15 PED
500 PED - 20.47 PED
700 PED - 26.39 PED
1,000 PED - 33.77 PED
1,500 PED - 43.26 PED
2,000 PED - 50.37 PED
3,000 PED - 60.31 PED
5,000 PED - 71.67 PED
7,000 PED - 77.97 PED
10,000 PED - 83.48 PED
20,000 PED - 90.99 PED
100,000 PED - 98.05 PED
9,999,999 PED - 99.98 PED
===
Code:
Markup - Auction Fee
0 PED - 0.5 PED Markup Taxes on shops tax markup / auction markup
5 PED - 0.74 PED +5 0.12 0.162162162
10 PED - 0.99 PED +10 0.25 0.252525253
20 PED - 1.49 PED +20 0.50 0.33557047
30 PED - 1.97 PED +30 0.74 0.375634518
50 PED - 2.93 PED +50 1.22 0.416382253
70 PED - 3.88 PED +70 1.69 0.43556701
100 PED - 5.26 PED +100 2.38 0.452471483
150 PED - 7.47 PED +150 3.49 0.467202142
200 PED - 9.58 PED +200 4.54 0.473903967
300 PED - 13.53 PED +300 6.52 0.481892092
400 PED - 17.15 PED +400 8.33 0.485714286
500 PED - 20.47 PED +500 9.99 0.488031265
700 PED - 26.39 PED +700 12.95 0.49071618
1,000 PED - 33.77 PED +1000 16.64 0.49274504
1,500 PED - 43.26 PED +1500 21.38 0.494220989
2,000 PED - 50.37 PED +2000 24.94 0.495135994
3,000 PED - 60.31 PED +3000 29.91 0.495937655
5,000 PED - 71.67 PED +5000 35.59 0.496581554
7,000 PED - 77.97 PED +7000 38.74 0.496857766
10,000 PED - 83.48 PED +10000 41.49 0.497005271
20,000 PED - 90.99 PED +20000 45.25 0.497307396
100,000 PED - 98.05 PED +100000 48.78 0.497501275
9,999,999 PED - 99.98 PED +9999999 49.74 0.4974995
So as you start out at +1 Shops save you roughly 16% as a shop owner vs being an auction seller... As that number grows you start to approach 50% savings, but never quite make a full 50% savings. If these calculations are right, it just goes to show that MA|Planet Partner gets about 2x more out of it's cut of auction sales than Land Owners get ouf of shop taxes, and shows that it's cheaper, in the long run, for shop owners to sell in shops than in auction, assuming they can get regular repeat customers.... which isn't as easy to do as it might sound.