Debating the Tobin Tax

I have discovered a market trader blog, (Long&Wrong) who is, amazingly, a Green Party member, and we are debating the Transaction (Tobin) Tax.
Politely.
Just in case anyone is interested.
He claims that the miniscule tax on each transaction would increase his trading cost by 300%, and his costs are 10-15% of his profits.

Hence the squeals of pain from the traders over what seems a miniscule tax. The do a lot of transactions.

I'm not defending the traders, I am signed up to the Robin Hood, but just filling in a bit more of the picture.