Calculation of Forex transaction costs

I am analyzing some Forex transaction data from the API and we know it doesn’t take into consideration transaction fees. I have the actual data so I am going to manipulate it.

My broker currently charges $2.50 per contract per trade and I am using that in my analysis.

Aside from a flat fee, does anyone recommend another way to calculate transaction costs for Forex? Is $2.50 reasonable? I know other brokers charge differently. I know this is a simple question. I am just covering my bases.