Last time I used the 'on demand' roaming in the US the rate was $7/day when/if used. When you reached $50 that was the max charged to roam for that billing period.
Before I use the service again, I checked to make sure all was good to turn the service on again. To my dismay, the rate is now $8/day and you can go up to $100 before the max for that billing period is reached.
Typical I suppose, that greed is ever present and they will charge whatever the market will bear!