That seemed much too high, since I have heard people playing silkroad on a dial up modem! Also, my connection can dip below 30kb/s down and 15-20kb/s up and I can still play SRO fine.
I just tested it with
Bitmeter II, and it says when using just silkroad online, i used about:
218 bytes down a second
145 bytes up a second.
(I had it run two minutes while I frolicked about killing things and running about. it worked out to
33.14kb down
17.71kb up
Thats total, not per second,

.)
That doesn't seem like enough, but the meter seems to be working fine. Surfing about on this site for one minute causes it to to register...
2.68MB down
130kB up
If my figures are right*, you can play silkroad online on a 5gig cap 24/7 for about... 5 or 6 days, lol. Or considerably less if you do anything else online!
*The probably aren't, hahah.