Fish Finder

 i know a similar topic was brought up about buying a depth finder for $200.  i need a fish finder for the back of the boat.  i already have a garmin 240 on the front of the boat and have been pleased with it.  my boat doesnt have a console i just sit in the back and hand steer the 20hp merc.  i was looking at basspro and saw several different finders.  i did look at the fisheagle 480 and was impressed.  im not sure if i can spend $200.  so the question is should i just get one for around $100 since it will be on the back or should i just spend the money and get a better one.