Lessee... if I don't take into account new models, stuff I give away, etc... and just stick to juice, replacement batteries, cartos...
Hmmm... I used to spend about $180/month on tobacco.
I just made a 30ml bottle of juice last night, and I figure the cost at $4.50 for the nicotine, $1.39 in flavoring, and maybe a buck for the PG/VG... let's call it $7. I'll go through about 3ml/day, so three of these bottles for the month = $21
A 5-pack of cartos lasts me a month, so let's say I get the $9 box for that.
Set aside enough for one battery a month ($5.29 for a two pack from DX), so let's say $3 for the battery allowance.
...and then add in $10 for fudge factor & parts allowance if I have to build a new battery mod if this one I like breaks (total parts cost to build it was about $15 tops)

...
$21+9+3+10=$43 per month expenses.
Ergo, I'm saving $137 per month, approximately. That is, I would be if I just stuck to the minimum necessary to vape. I think I'll go cruise the suppliers forums now and do some shopping....
sigh...