This is what I've been running since 2004:
http://www.lowrance.com/en/Products/Marine/Compact-Sonar-Fishfinders/M68C-SMap/
Lowrance M68C S-Map. Small unit, very sensitive and big enough to see from the bow. Shows all the pilings (nice for running at night) as well as your plotted trail.
I've been running the same head unit on well....a bunch of hulls since I bought it. I picked up a few transducers and just install one on each hull I've had or taken the transducer off the old hull before I sold it. I use the PRDT-WSU shoot-through hull transducers so there is nothing hanging off the back.
The head unit remains in the back of my truck when it's not on a boat, and has been beat up, dropped, stored in the car with the windows rolled up, etc. and it keeps on ticking.
I think you should easily be able to find one within your price range
-T