I suppose the answer is that it completely depends on a lot of things.
How much you smoked and what set up you get to start with. You can get setup for as little as around US$25-30 but that is going to be a basic kit and will last a couple of weeks with no extras.
Then you have to find a juice you like, juice starts at around US$5 for 50ml (for a Dekang) and you'll probably vape about 3ml a day - so that's about US$2 for juice a week and then supplies for your equipment - say another $1.50 a week for replacement coils.
So, in theory, you can get going and then maintain yourself for less than US$5 a week. The problem comes when it turns into a hobby, and you start wanting the latest products, different juice and the like.
I spent around US$8-900 to get going over the first 4 months or so, but now I have plenty of equipment, juice and batteries. So in the last 3 months, I'd guess I've spent less than $150 - all on juice. i could easily not spend anything from now on and last months and months with not spending anything. I have backups, extra juice, flavoring, wire, wick, nicotine base, batteries, chargers etc, etc.
You can also start making your own juice, repairing your own equipment etc to further reduce the cost.
In summary, yes it can be less expensive than smoking, but it often isn't.