let me see if I can explain it using fake easy numbers to explain the way i understand profit and spread and other fees.
Some exchanges just flat out charge per trade on every trade. I am seeing this normally at about .25% per trade.
Some exchanges do a Taker and Maker structure with their fees. Takers (people who do market orders) pay a .3% fee, and Makers (those who do limit orders) pay 0% fee
Then there is the price difference between the bid and ask. Depending on volume, the exchange used the difference between the bid and ask can be 0-5%
Say I am buying something that has spread of 2%. Does Gunbot sell 2% higher than when I bought it, or does it know there was a big spread so I would actually need to sell at 4% to make 2% profit.