A company manufactures a product with the following standard costs per unit: Direct materials $20, Direct labor $30, and Variable overhead $10. The company produced 10,000 units during the month, incurring actual costs of $210,000 for direct materials, $290,000 for direct labor, and $100,000 for variable overhead. Calculate the direct materials price variance, direct labor efficiency variance, and variable overhead spending variance. Discuss the significance of each variance and potential causes of unfavorable variances.