what's the average cost of a hospital that includes a drug called pravastatin since 2101?
select avg(t1.c1) from ( select sum(cost.cost) as c1 from cost where cost.hadm_id in ( select prescriptions.hadm_id from prescriptions where prescriptions.drug = 'pravastatin' ) and strftime('%y',cost.chargetime) >= '2101' group by cost.hadm_id ) as t1