Chris: This response may be late, but I hope it will help anyway. All labor burdens are not created equal. Look at FAR 31 and you'll get a glimpse of the variety of indirect cost factors that MAY go into the determination of the burden that gets stacked on top of the direct raw hourly wage paid to the employee. You get the labor rate, then you get the labor burden, then you put the two together. Two different employers may pay the same hour wage to their employees of the same class, but have drastically different labor burdens. For example, one employer may have higher overhead due to offices in a high rent district and high insurance premiums derived from bad workers comp history, while another employer in the same industry may have low overhead due to subsidies recieved from some local economic development group with low insurance premiums.
I would recommend that you shift your seach to what is an appropriate labor rate. Try checking the Bureau of Labor Statistics (http://www.bls.gov/bls/blswage.htm). In addition to a host of other data, they maintain wage data by labor categories and regions. Use this data to help you figure whether the base hourly wage is too high/low. From there start thinking about whether the burden rate is appropriate. If you are concerned that the burden rate is too high, have the contractor (or better yet their comptroller or accountant) sit down with you and have them walk you through their labor burden calculations while you have FAR 31 sitting in front of you. Have them explain how each line item in their calculations is allocable and allowable. You may find that the burden rate comes down.
I would also caution against reliance on historical pricing. "Hey that's what we paid before" presumes the last contracting person or team engaged in appropriate pricing practices. It also neglects the maxim that markets fluctuate. I'd love to be in a position to pay what I paid for gas last month, but not what I paid last year.