The rate I was quoted by AT&T for international data roaming in the UK was $0.0195/kb. So given my usage, without the plan I'm looking at a $400 for the 8 days I'm gone whereas the plan lowers it to $24.99.
Furthermore, the customer service rep said both the global voice and data packages are pro-rated by the day. So, if I activate them in the airport on Saturday and then cancel them on the Sunday I return I'll only pay about $6.66 (($24.99 / 30 days)* 8 days of usage) for the data and $1.59 for the voice.