The auto units that have comparable features to the MC units are also comparably priced.
The Garmin units below $400-$500 will not allow pre-programmed routes to be uploaded, and they didn't used to allow more than one "via" point. Some of the newer ones allow for up to 5 via points, which may or may not be enough to allow you to program the route that you really want... depending on how many returns to the interstate your route involves.
Setting avoidances to "avoid highways" is not always the way to go... one rider was riding from Dallas to Atlanta, and he wanted to take the "scenic route" so he set "avoid highways"... the GPS tried to take him from Dallas to Atlanta on the frontage road
The cheaper units are great for getting you from point A to point B as quickly as possible. Most now have internal MP3 players, but when you combine their limitations with the need to get off the road and secure the unit at the first sign of bad weather really hurts their usefulness, plus the Garmin Zumo series includes a RAM mount to securely mount the unit to the bike with a setscrew on the release so it cannot fall out of it's cradle. That's a $40-$70 value right there.
On the Zumo series, you can also tell it what your fuel range is, and when you get within, IIRC, 30 miles of the set range, it displays a "low fuel" warning. Clear that and a gas pump icon appears on the screen in the bottom right corner. Tap the fuel pump and it will give you a list of the nearest gas stations in it's database.