Medical Helium is not used to inflate balloons. Supagas, a leading supplier of helium explains that the helium used to inflate balloons is not pure enough for medical or scientific use. It is a by-product gathered during the production of medical grade helium. The majority of liquid helium boils off in the transfer process.
Instead of wasting this resource, it is collected and recycled. The sale of balloon gas offsets the cost of medical helium.