I am runing about 75+ mil per month and this figure is rising as I find flavors I like. This cost, about 50 cents a mil in my case, is soon to exceed my former tobacco costs so I will have to look into the DIY area soon.
Here's the easiest DIY to save a couple of dollars: If your juice vendors offer to double the flavor opt for that and order it in the highest mg they have, then you can dilute it to whatever mg you like (as close to half as possible) without loosing the flavor using either VG or PG which is relatively inexpensive.
In this way I can buy 120ml of 36mg Wyatt earp at freedomsmokes and dilute it (using PG, which is cheap) to either 18mg or 24mg.
The 120ml costs me $41 but if I dilute it to 18mg then I end up with 240ml of it for only $41 and the cost of 120ml of PG (which is cheap). 240ml will last me 2.5 months -- way less than I was spending on cigs, that's like $4 a week.
Sometimes I'll mess around with some samples or flavorings, but when I find one I like I'll buy it this way and double it.