Cost of depth sounders vs fishfinders
Check out the transducers that come with each unit. Many Fishfinders use transom mount version versus the thru hull transducers used most often in sailboats. A transom mount transducer is not a viable option on a sailboat for many reason one of which is that it comes out of the water when heeling.
My boat came with a fishfinder that had an optional thru hull transducer available. When I enquired on the price of obtaining the thru hull transducer I noticed that for very few dollars more I could buy a whole new depthsounder (not fishfinder).