I had the same predicament when we bought our C 30.
The depth sounder crapped out. You can have older 4" round type units such as Datamarine repaired by marine electronic shops, or you can by them new for about $250-$300. for the readout. But you can also buy the small puck size sounders for only around a hundred bucks from West Marine, etc. or online.
I also went with the option of a new chart plotter & sounder combo unit, as the money spent tackles 2 birds with 1 stone. The Garmin 440s that I have is a great 4" chart plotter unit for around $400. & an extra $100 buys the sounder unit that is plug & play, sounds through the hull, so requires no drilling or permanent fitting of the transducer, like the older models. For the money, its hard to beat & us a good investment for the boat, for the day that you go to sell it, & I think a GPS unit is really a necessary piece of hardware for safety. My 2 cents.