there are some free options. Google for com0com & hub4com, made by the same guy. I've bumped into some others too in the past but don't remember their names off the top of my head. Hub4com seemed good, which is why I remembered it.
Whether it works with your station depends on whether the station's serial protocol is send only or send and receive. If it is send only (the station just sends data at regular intervals without needing or expecting to receive commands from the connected program), then a simple software serial splitter (like gpsgate or hub4com) should work fine. If the station has a two way protocol, then you'll likely need some software to act as a broker/multiplexer, like what VirtualVP does for the Vantage Pro.
Steve
SoftWx