That depends on a lot of things. Which Flavourings you use (some require 2% others require 30%), where you get nicotine/pg/vg/empties/syringes, etc from
My more simple mixes cost about 50p per 10ml.. The more complex, high flavouring ones cost a little more than 90p per 10ml.
Yep it's cost effective if you're careful and don't end up wasting most your ingredients.
Once you have all of your gear, the raw materials cost, very approximately, £6 - £10 per 100ml or 60 pence - £1.00 for 10ml. Includes "Free P&P" of course!
Last time I did the sums I think it came out about 50p per 10ml.
If you are careful you could match this.
I like less flavour than most people at about 5% or 6% while many people use about 20%.
Most of the flavours I use are kandi hed from justvape24/7 at £2.10 for 10ml cheaper than most.
If you do not go overboard buying flavours you do not use and you build up your flavours gradually rather than diving in with overambitious plans (this can result in wasted juice)
It is well worth getting into.
I just bought 250ml mtbaker concentrates for £37 including postage, that will give me 1.25 liter of finished ejuice at 20% flavour well worth it. So you can have top notch concentrates and complex flavours and it still costs peanuts