After promising their new distributor they could produce 4 pounds of meth, Jesse starts freaking out. When the original deal was 2 pounds, Jesse was concerned about being able to buy enough pseudoephedrine to produce that. After showing up at their earlier meeting with only half of a pound, it seems impossible that the two of them can make 4 pounds weekly. It turns out that Walter can chemically create the same effect, but he needs Jesse to pick up some supplies. The elasticity of supply often dictates that the responsiveness of a good depends on how easily other substitutes can be acquired.
See more: inputs, Production, substitutes, supply elasticity, tradeoffs
The Economics of Breaking Bad 