depending on hours and condition, somewhere around 15k is reasonable & they are great boats, very well made, only a step behind mastercraft and nautique, but full composite hull and stringers should last a lifetime. the 99 as mentioned has the trunk and the Diamond sv23 hull, which is only slightly modified today on the limited build RLX (cut diamond) & They are only a hair behind the TXI for skiing and tracking still. one of the best inboard barefoot boats ever made as well... can't see if it has a wedge, but that can be added, which turns it into a capable wake boat.
he mentions a 351 motor in the ad, but most of the malibu's of that era came with 350s, check its not a typo, and what powerplant it has. There was a 350 carb'd 310hp, throttle body EFI, and MPI Monsoon motor. all are good, the EFI is likely the better buy for resale. never did have a problem with the carb'd 310 I had though.