Why not go UMTS? Such cards mostly support both: GPRS and UMTS and they are supposed to be macosx compatible. At least I know of vodafone pcmcia cards to be mac compatible.
But maybe the cheapest way would be a bluetooth cell that is sharing the internetconnection to the powerbook. Also take a look at the gprs-package prices. 
You should avoid high transfers if your company or boss is not paying for it.
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iBook 600; 12''; 640mb; 8mb Rage; DVD-CDRW-Combo, 20GB
P4 1.6; 2x80GB Raid1 (file-server)
tiBook 1Ghz, Superdrive, 768MB, 64mb 9000, 60GB
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