True, I also agree that to relocate to those slots will cost more DM in a certain way. I don't know what formula to follow, but this will surely make more profit to GF. and it's ok.
Allow the relocation to any slot. The Planets on slots 1-3 and 13-15 with Planets on 4-12 when relocating. So the player can relocate any planet to any slot after reaching astrophysics level 8.