Jonathan Gruber has crunched numbers and determined that —

• If the mandate to purchase insurance is removed from the Patient Protection and Affordable Care Act, by 2019 average insurance premiums purchased through an exchange will be 27 percent higher than they would be otherwise.
• If the mandate is removed, by 2019 about 7 million formerly uninsured people will have insurance coverage, but with the mandate in place about 32 million formerly uninsured people will have insurance coverage. However, the cost of the bill would decline by only 25 percent, because the seven million would be disproportionately older and sicker.
• If the subsidies are removed as well as the mandate, by 2019 average insurance premiums will be 50 percent higher than they would be otherwise, and the bill would have no effect on the number of uninsured Americans.

There’s a PDF report with tables showing how changes in the bill will affect outcomes. It also explains why repealing one part of the bill effectively kicks the legs out from under the other parts.

See also Eztra Klein, “Economic security — and insecurity — in one graph.”