> Cost of recruiting drivers is relatively high - probably around $50-$100 per driver.
Hrm. Instead of working from first principles, we can poke at some data. Zen99 calculates that Uber seems to be spending some $1M/year on Craigslist ads for SF drivers alone [1], and as of the end of last year, they had about 16,000 drivers in SF [2]. Assuming generously that they recruited them all that year, that's about $62 on Craiglist advertising per driver, much higher than your $5-10 estimate.
There are also incentives like an unconditional $500 on your first ride [3]. In NYC, Uber also offers new drivers a free defensive driving class and doctor's examination at their offices [4], because they need those to get on the road. You will not find doctors to do exams for $2-5.
I would guess total driver acquisition cost, including first-trip incentives, is much more likely to be in the $1000-2000 range, at minimum. And we still haven't gotten to long-term subsidies like uberPOOL, which is what I'm really curious about. Even a $10K driver acquisition cost could maybe be sustainable if each driver lasts over a year on average, since it's a one-time cost. But it needs to pay better than taxi wages + cost of car maintenance for the driver, and at least break even for Uber, in the steady state.
[1] https://tryzen99.com/blog_posts/on-demand-is-in-demand
[2] http://www.washingtonpost.com/blogs/wonkblog/wp/2015/01/22/n...
[3] http://www.driveubernyc.com/try
[4] http://www.driveubernyc.com/defensive