set PROD; # products
set ACT; # activities
param cost {ACT} > 0; # cost per unit of each activity
param demand {PROD} >= 0; # units of demand for each product
param io {PROD,ACT} >= 0; # units of each product from
# 1 unit of each activity
var Price {i in PROD};
var Level {j in ACT};
subject to Pri_Compl {i in PROD}:
Price[i] >= 0 complements
sum {j in ACT} io[i,j] * Level[j] >= demand[i];
subject to Lev_Compl {j in ACT}:
Level[j] >= 0 complements
sum {i in PROD} Price[i] * io[i,j] <= cost[j];