Yes, even though the airport is only a 20-30 minute drive getting there can be expensive. There are a number of ways:
1. Get a friend to take you
2. Park your car there. I do this for weekend trips. One of the "off airport" lots is $11/day and has a buy one day get one free coupon.
3. Michigan Flyer which as you mentioned is pretty cheap. However, I've never done it because it leaves from where a lot of the hotels are near Briarwood which isn't very convenient.
4. Shuttles: There are a number of shuttles (like Super Shuttle) that do door to door service and pick up others on the way to the airport. These run about $40 each way for one person and go down per person the more people you have going from one location.
5. Air Bus: This is an option during university wide holidays only. Basically Michigan runs a bus from central campus to and from the airport during the most popular times to leave. Cost is about $8 each way. They will sell out on popular days/times and you have to physically go to the Union to buy a ticket. But if its the right time for you the price is great. I did this when leaving for Spring Break.
6. Taxi: This is the most convenient but most expensive. Generally around $60 each way.