Under the current design of PUT charges the incentive would be in “PUT” balance. So 100 puts for a file and get 10 puts added back to the balance.
Doing it in coins would see people treat the PUT cost algorithm as a “stock market”, or even be able to game it.
Say big player puts up terra bytes while PUT cost is extremely low, like one coin per 100 terra bytes (put cost can be lower can be lower @ 2^63 puts per coin). Then waits for network to be loaded and put cost is one coin per GB. So if you gave back 1/10 of the put cost in coins then they could get 10000x the coins (100TB/1GB=100000 & 1/10 = 10000 coins)
So for their one coin to upload they get 10 thousand coins.
So if a really big player got in early and offered 1000 TB in vaults and stored 100TB while the cost is 2^63 puts per coin, then later on when SAFE starts to fill up they remove their storage and the PUT cost rises a lot then delete their 100TB and winfall profit.
But if you only gave them put balance then all they get back is 1/10 of their PUTs done, so really 1/10 of what they had when they uploaded the 100TB, IE they lose if they tried that.
tl;dr
You cannot give coins in return for the delete incentive, the system cannot keep track of the coin cost to PUT data. The PUT balance can be bought at different times and at different costs. So best to just return a %age of deleted to their PUT balance