Assumptions:
24 Mbps / 8 bits = 3 MB/s (this is the tier I am on, and I can get around 2.8MB/s download, so this is close enough).

Caps:
150 GB * 1024 KB = 153,600 MB (DSL)
250 GB * 1024 KB = 256,000 MB (U-verse)

Formula: (using U-verse 24 Mbps)
250 GB * 1024 KB * 8 bits / 24 Mbps / 60 sec / 60 min = 23.7 hours (1 day) to hit cap.

I got the tiers from: http://www.att.net/speedtiers

Results: (time to reach cap at full throttle)
455.1 hours (19 days) for DSL 768 Kbps
227.5 hours (10 days) for DSL 1.5 Mbps
113.7 hours (5 days) for DSL 3 Mbps
56.9 hours (2 days) for DSL 6 Mbps
189.6 hours (8 days) for U-verse 3 Mbps
94.8 hours (4 days) for U-verse 6 Mbps
47.4 hours (2 days) for U-verse 12 Mbps
31.6 hours (1 day) for U-verse 18 Mbps
23.7 hours (< 1 day) for U-verse 24 Mbps

Now, if all of the above were paying the same, I might see this as fair, but considering that the faster speeds pay more, but do not get more cap, this is absolutely NOT fair. For some more math fun, how much could you download in 30 days on the 24Mbps plan?

Answer: 7,593.75 GB. Overage fee @ $10 per 50GB would be: $1,470.