Tradehill always takes their fee in USD, likely so they don't make half their profit in Bitcoin, and have to deal with trading themselves and accounting weirdness this would cause a business.

If you buy dollars, you spend the amount in bitcoins you specify, and your fee is taken from the dollars you bought. If you buy bitcoins with dollars, you must pay additional dollars as a fee.

Obtained Amount	Obtained Currency	Paid Amount	Paid Currency	Commission Amount	Commission Currency	Final_Obtained Amount	Final_Obtained Currency	Final_Paid Amount	Final_Paid Currency
27.2160000000	USD	2.0000000000	BTC	0.1469664000	USD	27.0690336000	USD	2.0000000000	BTC
1.0000000000	BTC	13.0000000000	USD	0.0702000000	USD	1.0000000000	BTC	13.0702000000	USD

(this is with a commission rate of 0.54% - 0.54% of $27.216 is $0.1469664; 0.54% of $13.00 is $0.0702)

The beauty of exchanges is they charge both the seller and buyer a fee for the currency exchange, in essence taxing the same transaction twice.