Did know that either. I've spent over $16M on ships and upgrades since leaving Founder's World. I could have got the same for only (ONLY!!!!) $14.6M.
I feel cheated somehow. Waah
Go back to founders world, check which of your new equipment they have in stock (they dont have 6A shields and 5A FSD for example), sell those what they have in stock for a lower price one. Leave the syetem for a mission from the bulletin board or just jumt to somewhere and back. Dock. Buy back your equipment with a lowered price by 10%.
When you sell your equpment the price will be the same as you bought it no matter they have it lower in there. Same with equipment bought there, you can only sell it in other systems on the 10% discounted price. Otherwise people could exploit it.
So just go there and get some of your money back. I did this all the time and it works.