EDIT: Thank you to everybody that voted and commented. Change all you want now!
I'm not entirely sure why metamorphosis has a cost, but this is why I think it might:
1. To discourage people from continuously changing their mark
2. ???
3. Profit.
Now, it makes sense that you don't want newbies to get carried away changing their deck continuously, so that's why I suggest the following:
*every time you use metamorphosis, it becomes 1 cheaper*
This way, you'll spend a total of 5,000 (EDIT: 5,050) coins on metamorphosis, which you wouldn't even be able to accumulate until you've played for a fair while.
This would prevent people from going crazy with mark-changing when beginning to play, but being able to experiment/play around with it freely after paying 5k over time.
Vote above, comment below!