Bob;
if you google "exchange pound to usd" it should come up with some on-line calculators. I just did the same for Canadian dollar last week when I renewed.
Now, on your credit card bill, the actual daily exchange (and possible fees) will make it slightly different from the on-line calculators, so just treat the on-line calculator as a close but rough guide.
For instance, 108 pounds comes out to about 142 US Dollars, as of the time of writing this reply. Hope this helps and adds to Jason's answer above.
John.