My preferences for navigational devices:
$20 App on the phone (Navionics or others, always good to have, difficult to use on the water)
$50 App on the iPad (no real benefit to the phone, same difficulty to use)
$240 Garmin 44cv series (sonar+chartplotter, excellent, you get depth plus nmea/AIS)
$260 Garmin 54cv series (same, as above, larger screen)
$799 Raymarine Axion 7 (full featured chartplotter+radar display)
$999 Garmin/Lowrance/Raymarine - lots of choice
I would no longer consider the Garmin 72/76/78 series good value. Small screen, odd shape, difficult to read. You can also look at eBay where decent devices can be had for less money. One way, to think about the chartplotter as a system is that you can save on other devices that cost money. For example, if you get the Garmin echo sounders above you get a very capable depth sounder that you can mount in hull and avoid the pain of installing a separate depth meter. Similarly, if you attach a nmea compass to the Garmin or the Raymarine, you do not need a compass at all (you can have a great fluxgate compass for $99 which is superior to any analog compass.
SV Pizzazz