While these riders at the Capital City Cycling Classic Criterium on Saturday likely spent thousand of dollars on their bikes, you don't have to spend nearly as much to find a great bike to ride in Lansing. On the other hand, I have had a few friends recently ask me where they can buy a great bike for $100. My response is to them is Craigslist, but using Craigslist requires being knowledgeable about bikes to find a good deal. My wife and I currently own eight bikes. The road bikes were bought from bike shops. The two mountain bikes were hand-me-downs from a friend who was upgrading his mountain bike fleet. Another bike was bought for $120 on Craiglist and is my daily commuter. Another was bought from Share-a-bike in East Lansing and another from Kid's Bike Repair in Lansing for $20 (this will eventually be a bike polo bike and required a lot of work). The last is my wife's father's bike from college. The road bikes were purchased when we knew little about maintaining and repairing bikes and also didn't have many connections in the local bike community. However, as we've learned more about bikes, we've tended to stick to the used market to find good deals on solid bikes.
Please don't buy a bike at a department store. There are higher quality bikes available for similar prices elsewhere. Many people will head to a department store (Walmart, Target, Meijer) to buy their first bike. These stores indeed have bikes to buy that are relatively inexpensive ($100-$300) but are of questionable quality. These bikes are typically referred to as Bike Shaped Objects. They are typically assembled without care and their non-standard parts can lead to maintenance issues down the road.
If you don't know much about bikes, I suggest first visiting a bike shop. There are many in the local area staffed by knowledgeable and friendly people. When you walk in, they will ask what type of riding you'll be doing, and how much you're looking to spend. They will be able to tell you what size of bike you'll need and sizing is very important when it comes to riding a bike. A poorly sized bike will not be comfortable and you will not want to spend much time on it. The cost of adult bikes at a bike shop will range from $300 to more than $5,000. Most bikes purchased at a local shop will also come with free tune-ups for as long as you own the bike. The local shops in alphabetical order: American Cycle and Fitness, Denny's Central Park Bicycles, Evergreen Cycles and Repair, MSU Bikes, Riverfront Cycles, Spin Cycles, and Velocipede Peddler.
Kid's Bike Repair, Lansing Bicycle Co-op, and Share-a-Bike
These three groups have different missions, but all include repairing donated bikes and selling them to help fund their operating costs. There groups are full of volunteers who take used bikes (some with more than few issues) and use their expertise and supply of parts to bring these bikes back to riding shape. Kid's Bike Repair has the largest facility on the south side of Lansing. Expect to pay between $100 and $300 for a bike. The price might be the same as a brand new department store bike, but most of the available bikes are are superior quality and have been assembled and tuned by folks who enjoy bicycling and want others to share in the fun. These groups are a great place to make connections in the local bicycle community. The Lansing Bike Co-op has open shop on Wednesday evenings where you can bring your bike in and learn to fix it with the help of the volunteers.
Craigslist is the holy grail of the used bike market, but I would suggest avoiding it if you are new to bicycling. The Lansing bicycle section has healthy supply of new ads posted daily of bikes ranging in price from $10 to thousands of dollars for a used high-end road or mountain bike. There is a lot of variability in the quality as well and you should know what you're looking for when you go to take a bike for a spin. Try to bring someone along who knows what to look for (or post a link below and we can offer some pointers).
So where can you buy a bike for $100? Your best bet is Craigslist, but you'll need to know what you're looking for. You won't be able to buy a brand new, high-quality bike for $100 or even $200. You'll likely end up spending a bit more if you head to a bike shop but you'll get a well-fitted and tuned bike. If you spend $800 on a great bike, the healthy used market might allow you to sell it for $600 a few years from now if you end up upgrading or finding a new mode of transportation or hobby.