Overhead, overhead, overhead. Something that you'll love and hate at the same time. The average amongst offices in my area (Southeastern New England) based on informal discussions with my colleagues is in the 55 to 65% range. I've heard of ranges from 30% all the way upto 85%
Things to remember though about overhead: You need to look at the fixed cost side(rent, equipment purcahses, salaries {basically these are fixed costs}, insurance for the practice, liscense fees, etc) verses the variable expenses (supplies, lab fees, continuing education costs, equipment purchases, etc). Notice that equipment purchases are both a fixed cost (paying off already bought equipment) and a variable cost (buying new "toys" for the office).
Last year, my office had an overhead of 53.6%, this year I'll almost guarentee that we'll be above 60% (we just bought a soft tissue Nd:Yag laser (chalk up $27,000+ to the varibale expenses, and a bleaching light ($3000) and we finished off equiping the new operatory that we built the end of last year add another $10000 to the variable side) This will drive up our overhead for this year, but these are costs that won't be occurring every year. As a rule of thumb if your looking at a progressive office, you'll have a higher overhead since new technology is always being incorporated into the office. Offices with lower overhead may not have as large an investment and interest in new technology.