Finding the cheapest way to mix your oun is only part of the problem (part of the fun?).
Read up on DIY mixing, read a tutorial or two, practice with one of the on-line calculators.
Being able to calculate the final strength of a mix, as well as PG/VG percentages will be necessary to know what you are going to end up with, before you end up with it
Get the math down before making your purchases.
Another issue is that a lot of flavors are PG based. If you use 10% flavor about 7% of that is PG.
Practice the math, then select your vendor for the base Nic.
Start using mg/ml for strength, and ml for volume. It will help later on to keep things straight.
60mg/ml nic base in 100%VG, and some PG would be an easy route to take.
Twice as much PG as the 60mg/ml Nic/VG base would give you 66% PG, 31 % VG, for a strength of 20mg/ml. Add 10% flavor and you're about right on your target.
Have fun, be safe.