Fish swim around, school, fight, defend territory, woo each other, breed, care for babies... AND they are pretty. Most fish in your average LFS can be kept in 20g with a HOB, no light and dechlorinated tapwater. Some you can keep in a 5g.
Reptiles (and mammals) are expensive, smell bad, and most of the time sit still. I had a turtle for a while, and I gave him to a woman who was very happy to take him. I had fun, and for the first day, they are more fun then fish, but it just really either sat in the water with its nose out, or in the heat lamp. BORING. (you also need a filter, and a lot more waterchanges then for fish.) Most reptiles are like this. Mammals like guinea pigs are just as boring, but they are noisy and stink too.
Also you are forgetting the exotic and often live food you need, and how often you have to change bedding/water.
If you want low maintenance and cheap, collect posters.