“As we have been unable to agree on salary for the position, it is with regret that I will be declining the offer. I’d like to thank everyone involved in the interview for their efforts. Regards Force_maj”

Don’t apologise; they also wasted your time; they know what a reasonable salary is and they’re assholes for not offering it.

e: reverse the opening line: “It is with regret that I will be declining the offer as we are unable to agree on the appropriate salary for the position.”