All we need is for the value of the highest fees in the last block, plus the highest fees in the mempool to exceed the block subsidy. At that point, you get more sats from reorging the last block than you do from simply advancing the chain.
It's not that simple.
If you are reorging the chain AND rest/most/significant_part of the network mines on-top of the last block; then your threshold for profitability of such action is significantly increased.
Effectively it means that you need to mine >1 successive blocks if you want to do the reorg. That changes the profitability equation a lot because of geometric scaling of hitting successive-blocks mined.