The answer to your question depends on the contractual arrangement between you, the staffing agency, and the employer. Is there a provision in your agreement with the staffing agency to accept minimum wage for resignations without notice?
In general, an employer may not unilaterally change your rate of pay without notice, and may not do so retroactively. Absent an agreement to the contrary, the hours you worked should be compensated at your agreed-upon rate of pay ($34.50), even if you did not provide notice. You should consult with an Atlanta-based attorney to review the procedure for recovering the balance of your wages.