I was riding on a taxi back from the new airport when I noticed that I was being charged 2 kuai for every unit. After the fare rose past 30 kuai, I started getting charged 3 kuai per unit.

I guess price rises are inevitable but this seems a little steep. Or maybe it's only airport taxis that charge so much?

Yeah, it was a fancy black cab. There were some standard blue Toyota taxis there but most of them were the black ones.

I've learnt another lesson in China. Not all cabs are created equally!

I went in the same taxi once... Difficult to stop the taxi when you discover that after it started (thankfully my company was paying...... but that's not the point).
Do you think it's legal, or do we need to report them to the taxi authorities. I mean, they have real invoice papers, so they must be some kind of legal business.

Btw, I was quite displeased to find that the price for the shuttle bus to Beijing airport has also gone up 10 kuai to 80RMB, although the vehicles themselves remained the same old, dirty, and totally uncomfortable ones. Even funnier if you consider that the fare from Beijing Capital Airport to the Air China ticket office on Nanjing Lu is still 70 RMB.   

Yeah, Sandiego, I was told this as well. But if the taxi is running a meter wouldn't it still be the same?

 I would have taken a ride in one of those black vw santanas as well from the airport, if someone hadn't dragged me away. The taxi driver explained to me some sort of arrangement between the car clubs and the airport, to ensure late flight passengers would get a ride into town. I doubt they're illegal, just pricey as admin said they usually hang around the 5star hotels

After 15 or 20 Km (don't quite remember) the per KM rate rises from 1.7 to 2.6

ha ha - that's funny - I do the exact same thing - I usually don't even bother to look at the guys waiting around like vultures outside the train station.  Usually walk - just like you - five minutes from the station till I'm far enough away that no one's screaming at me to ride in their car, find a nice comfortable, legit cab, and ride home

no change of price for night trip in Tianjin....

It's 2.5/km after 10 km. And every 5 minutes of driving (after the first 10 minutes i think), it goes up 1km price (so 1.7/5min before 10km, 2.5/5min after 10km). So no wonder it's cheaper if you take another taxi after 10 km, because you start again at 1.7Y/km during the first 10km.
However, when you go to the airport, it's a little complicated to ask the driver to drop you in the middle of highway after 10 km and wait for another one.
Last time, i went from the airport to Tianjin city (so more than 10km). I stopped by at a friend's place to put my bag and pick up someone to go by taxi to a restaurant. So i asked the driver to sum the bill.... and wait for me (so that i won't have to pay 2.5km for the following kilometers...) I felt a little 不好意思, but... well, when you can save a few yuan....