How much does the initial investment generally cost? I'd likely be making 70/30 to 80/30 VG to PG 30ml 6mg batches most of the time. I tend to lean more towards fruitier flavored juices, I like my juice to be simple but enjoyable. I can't stand how so many juices have 4-5+ flavors going on, what's the point?!
That depends on how much supplies you want. You could start small and get 32oz/1 liter bottles of Pg and VG for around $10 each. A 250ml bottle of 100mg nicotine solution would run around $20-$30. If you're just starting out and want to try a bunch of flavorings you can get 30ml bottles of flavoring concentrates for around $5-$10 each or go even smaller and get 4ml-10ml bottles for $2-$4.
Add in a half dozen of 10ml and 1ml blunt tip syringes and a half dozen bottles to mix the juice in and you're looking at a total investment of around $100 and will have enough to make around 2,500ml of juice. Your cost of production would be 25ml per $1 spent.
If you want to go big you can get 1 US gallon of both PG and VG for around $50 shipped and a 1 liter of 100mg nic base for around $50. That's enough base ingredients to make around 8,000ml of unflavored juice for $100. Then you just need the flavorings, syringes to measure with, and juice bottles to mix and store in. Figure $150 total investment for nearly 9,000ml of flavored juice, for a production costs around $1 per 60ml.
I went big, and even my most expensive mixes cost less than $6 per 250ml bottle to make.