forex backtesting spread cost modeling

External question — Quantitative Finance Stack Exchange Author: Igor Rivin Original post: https://quant.stackexchange.com/questions/41401 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. For professional level forex trading, what is a reasonable estimate of roundtrip transaction costs (I am talking about major pairs, like EUR/USD).