A typical going rate for repairs, when repairs are possible, usually starts at $50 plus parts, plus shipping.
While the 800xlt was a nice scanner in 1985 / 1986, its features are very limiting today.
You might find a used BC15X or similar on one of the online for-sale sites for about the same price as repairing the 800xlt, and gain the availability to trunk-track, extra memory, tone decode, alpha-numeric display, etc.