If you aren't too fussed about varying nicotine levels, and want max VG, the cheap, and still simple, way to do it IMO would be to buy a bottle of Darkstar nicotine (£16.99 for 250ml 72mg/ml), and a big bottle of lubrisolve VG (£16.99 for 5 litres). Then empty 250ml of the VG out into the bottle you'll have left over after this month, and pour the nicotine into the 5l jug, give a real good shake, and you'll have 5 litres of 3.6mg for £34. Seems like a lot but you can just buy and forget about it for a couple of years then. Just shake, and refill your 250ml bottle with the VG/nic mix when it gets empty - this is much easier than trying to take a little bit out of the big jug, and it stops you opening the jug up loads of times so the nic in the jug won't oxidize as much. I personally don't do this because I'm mixing for friends too, so need to have seperate nic and base, as well as varying PG/VG ratios.
Then the only cost is your concentrates. I've only tried two brands, TFA and FA - FA is definitely cheaper, since the 1.5x cost is offset by the fact that most flavours are 3x as potent and FA's site offers six 10ml concentrates for the price of five, however I have found they require a little longer to steep, TFA is definitely easier to work with. Naturally if you find a flavour you love, you can bulk buy concentrates too to really bring your costs down.
One thing I will say is don't be put off if your mixes don't taste great at first, if you're mixing max VG, in my experience *all* juices need a good steep to develop. I mix up 50ml bottles, but I have about 5 mixes on the go, this means I'm not slamming each one before it has a chance to develop, and I avoid getting too sick of a flavour. Currently have a black forest gateaux, banana choc, lychee milk and honey, banana cookie, and blueberry apple pie with cinnamon+custard.
I'll give them a try after I'm happy with my mixes 
