Conceptually, there are two transactions costs: 1. Brokerage commissions, and 2. Bid-ask spread. Both are related to which market the pairs belong to (stocks, bonds, options, the country that pairs are located in, etc.). In the U.S. the bid-ask spread in really fast markets (large capital stocks) is close to zero right now, but for other securities...it varies.
Open a trading account with direct market access (dma) at an international investment bank with good stock broking operation and guarantee a certain turn-over, you might get away with 1 to 2 basis points (i.e. 0.01% to 0.02% of stock price) per trade. You will then usually trade synthetically via swaps (no physical delivery). Advantage of this is saving potential stamp duties, taxes, etc. Next, you have to consider also bid-ask spread (i.e. the price for purchasing is higher than the price for selling a certain stock). This depends on the liquidity of the stock and of course the currency. For highly liquid blue-chip stocks which are members of a respecable market index (Dow Jones Industrial, DAX, FTSE100, CAC40,...) this might be a tenth of a cent or penny... Good luck with your strategy...
https://www.researchgate.net/publication/287162366_Transaction_Banking_Services_The_Case_of_Bangladesh
Book Transaction Banking Services: The Case of Bangladesh
Article Pairs trading under transaction costs using model predictive control