any fishfinder will work as long as you can mount the transducer so it get's down the hole and stays in the proper position. Granted, some finders are better then others but they will all do the trick. For years I used a Cuda 168 black and white finder....it's about the cheapest one you can buy at around $100. I've had a Lowrance x67c ice machine for 3 yrs and it is an awesome ice fishing finder. I was lucky enough to get mine on sale at the toronto sportsman show for $289.00.
Cruise the fishing boards/forums buy and sell areas, kijiji, and even eBay......you might find a good deal.