The cost structure for Uber drivers is very similar to taxi services and over time will approach them.
Except for the cost to the taxi operators for their medallion. Since there is a a limited number of medallions and you need one to operate they tend to get transferred at great cost. For example a quick Google query for cost of taxi medallion nyc tells us that the current cost is down to $840,000 from a peak of $1.05 million in June 2013.
So the major cost of operation becomes the cost of financing the medallion. In fact (again according to Google) in most instances the medallions are owned by investment companies and leased to actual drivers.
Uber exists to disrupt the requirement for the medallions. They provide a lower friction billing system that makes it easier for both users and drivers to participate.
The ONLY people who are against Uber in the long term are the current owners of medallions. If Uber succeeds their investments will be valueless.