Viagra is produced in 25mg, 50mg, and 100mg versions. This makes sense, since most people only need a 25mg or 50mg dose. 100mg is too large of a dose for most people, and too high of a dose is more likely to cause side effects like headaches and flushing.
So why do online pharmacies usually sell the drug only in 100mg form? That's because customers can easily split the pill into two, four, or even eight pieces. This saves a lot of money over purchasing the 50mg or 25mg dose. Pharmaceuticals are generally sold by the pill, not by the dosage.
This also explains why online pharmacies often advertise their price "per dose" rather than emphasizing "per pill."
If you are interested in splitting pills, you might want to check out our review of pill splitters for ideas on how to pick and use one.