Nothing cheap. Garmin, Raymarine, Furuno et all worked hard to prevent that. Now if only Heathkit was still around, then we might have an option.
Cruzpro has a Box/transducer set-up that will hook into your laptop. Last time I looked it was about $800 for 320W peak-to-peak, 50/200. Kinda pricey for that kind of performance. Some of the higher end units do have a video output....but at that point you have already bought the high resolution display.
Seems kinda strange that I can buy a small screen DVD player at Walmart for about 100 bucks, w/ much better resolution than a $1200+ fishfinder AND with the old technology paper graph Lowrance 15 and 16's, I could darn near tell if the rocks on the bottom were round or pointy. But nobody can get that kind of combined performance these days for less than the price of a used car