I would pay maybe £0.09 per SMS if I wasn't on contract. As it is, I am, but I still have to pay to get a delivery notification - that's £0.01 per message or £0.02 depending on your contract. I send maybe 100 messages per day on Whatsapp and probably about 3 texts.

Also, where I live (UK) literally everyone I know who has a smartphone is on Whatsapp. It's completely ubiquitous.