Your tax rate seems about right

The the FICA (Social Security / Medicare) tax rate, is 7.65 percent for employees and 15.30 percent for self-employed people. The rates are broken out as follows:
* 6.2% (Social Security portion) on earnings up to the maximum taxable amount ($106,800 in 2009)
* 1.45% (Medicare portion) on all earnings.

So your tax of $1,100 / $18,000 = 6.1 percent sounds about right for the Social Security tax.

However, you may essentially be eligible to get some of your FICA taxes "rebated" to you thru the Earned Income Credit. See here for details :
http://www.ssa.gov/policy/docs/progdesc/sspus/ernintax.pdf