Well, you have the sequence a bit screwed up…
The agent took his 20% from the gross, not the net. Then he deducted a pretty hefty amount for tax, but depending on the W-4 you filed (you had to have filed a W-4 form), and with FICA and Medicare taxes + the ‘employer’ portion of each, since you’re apparently self-employed… it’s not wholly unreasonable. The tax amount might seem like a lot, but if it’s a short-time employment situation, the tax withholding is higher. (It doesn’t affect the amount of tax that you’re liable for at the end of the year, in fact you might get a decent ‘return’ from your tax filing, but it’s not smart to give up the money and then have to file for it later. Better to pay less now, and a little bit more when you file.)
If you’re not working like that often, then file another W-4 form with lower withholding, which is a totally legal and aboveboard thing to do, so that your withholding won’t be so high next time.