In response to Meredith's request to the mothers among us to tote up the number of "billable hours" in a pregnancy : This sum seems inherently incalculable, not only because it would differ wildly and unforeseeably from person to person and pregnancy to pregnancy, but because the normal model of pay for work just doesn't apply to bearing a child for someone else in exchange for money. A closer model might be civil compensation, such as if someone were hurt in an accident and a jury had to calculate how much her comfort, freedom of movement, quality of life, etc. were affected in order to reach a settlement.
Also, of course, a surrogate is incurring risks to her own health, agreeing to go through the pain and potential injury of labor, so there are things you're being compensated for over and above an hourly wage. I agree that $20,000 seems low (especially if you figure in that pregnancy doesn't really last nine months, but closer to 10, and that there's a significant recovery period after giving birth). But dividing the total sum by the number of hours you're pregnant (or the number of those hours that are experienced as inconvenient) doesn't really get at the problem.