The Mishna says:
If one takes payment to ... give evidence, his evidence is void. (Bekhorot 4:6)
So if you are paid to testify, your testimony is invalid. Does this apply to paid "expert witnesses", who come to court and say things like "I am a doctor and I say the defendant doctor showed gross negligence in how he handled his patient." ?
According to the Rashba, his testimony is valid.
The Beis Yosef (Choshen Mishpat 28:22) writes:
כתב הרשב"א בתשובה על ההוא דהנוטל שכרו להעיד עדותו בטלה נ"ל שלא אמרו אלא בעדים שנעשו עדים כבר שהם מצווים להעיד ולא רצו להעיד עד שנטלו שכר ודומיא דנוטל שכרו לדון מצוה על ב"ד לדון בין איש לרעהו אבל מי שאינו מחוייב להעיד ונוטל שכר לילך ולהיות עד אינו בדין זה לפי דעתי
The Rashba writes in a teshuva regarding one who takes payment to testify, whose testimony is nullified: It appears to me that this is only said if these witnesses already witnessed and are able to testify now, but refuse to do so unless they receive payment, as this is comparable to one who demands payment to judge even though there is a mitzva for a court to judge [without payment]. However, one who is not obligated to testify [as he never witnessed anything], and takes payment to go and be a witness, is not included in this law.
The Rashba holds that the reason that witnesses cannot demand payment is because the Torah already obligates them to testify, and their unwillingness to fulfill their Torah obligation without payment disqualifies them as witnesses.
Therefore, when somebody is asked to witness something and then provide testimony for payment, he is not disqualified since at the time he was paid he was not yet obligated to testify in court (as he hadn't witnessed anything yet).
An expert witness never witnesses anything regarding the case; he simply come to court to provide beit din with a professional opinion on something out of the judges' area of expertise. There is no halacha directly obligating him to testify. Therefore, the Rashba would allow him to be paid.
(The simple understanding of why a paid witness is not believed is that we suspect him of lying, in which case an expert witness would also presumably not be believed {assuming he was paid by the plaintiff and not by the court}. However, see Ketzos HaChoshen 34:4 who proves that paid witnesses are not suspected of lying; there is a different reason that they can't testify{for our purposes, the Rashba above}.)
