Quick point: to be approved as a generic alternative to Rogaine (that is as an approved OTC hair loss treatment) it must be identical in chemical properties.
The Walmart brand of minoxidil is chemically *identical* to the "Rogaine" brand. FDA regulations on labeling mandate this.
Keep in mind the brand name is a marketing brand and does not indicate a chemical difference - in over the counter medications.
The price difference is reflected in the fact that generic companies dont have to do any of the research required to get a drug approved, dont defend themselves from bottom-feeding lawyers and don't spend a penny on marketing or product awareness. Pfizer spent hundreds of millions educating doctors and consumers about "Rogaine", creating product awareness.
As a professional in the pharmaceutical industry, I can assure you that minoxidil is minoxidil - as long as you buy it from any of the many, many reputable retailers.