I bought a new HDS-5 for my 16 ft lund this year and have not regretted a penny spent on it. I am dialing in to specific structure and when I get a strike, I can look down and estimate why the fish may have been around a certain bottom contour and circle back and hit it again with success.
I bought the older Navionics Chip (I think the previous version was 2008 but may have been 2009) to save some $$ as the HDS was over $600 and the chip was (I think) $150 for the latest but I got the older one for $100. Nice thing about Navionics is you can send in the chip and $50 and upgrade to the latest chip which I plan on for next spring.
While I would have loved to have gotten the HDS-7 or 8, the price was a few hundred more and thus out of my range and I only have a small side console so these bigger units would have essentially had to replace the windshield.
At this point, there may be some HDS units either used or on surplus sales for around $500 so I wouldn't discount them as too expensive until you have checked all options such as eBay or Craigs list.