When consumers and employers pick health plans, some increasingly are being offered a trade-off these days: They can get a hefty break on their premiums if they agree to pay more out-of-pocket when they use certain high-cost providers in their network or if they cut those providers out of their network altogether. Blue Cross Blue Shield of Massachusetts this year introduced a "Hospital Choice Cost-Share" option. It tacks on extra charges when patients get certain services at 15 hospitals that the insurer says have higher costs than other providers. Patients pay an extra $1,000 for inpatient care or outpatient surgery at one of these hospitals, for example, and an extra $450 for high-tech imaging services. Among the hospitals on the high-cost list are Harvard teaching hospitals Massachusetts General and Brigham and Women's in Boston as well as UMass Memorial Medical Center in Worcester.