If your juice was 12 mg, it would cost me about $8.00 or less to make 150 mL of juice. Of course, that does not include the cost of buying flavors you do not like, buying dropper bottles, syringes, and other equipment. Or the time of making recipes that don't work for you. Making a juice is easy. Making a GOOD juice is harder.
As far as retaining equal to pre-mixed juices, it depends on the juice, how complicated it is (some are crazy involved to clone), and how much you care to be equal to pre-mixed. I have bought very few pre-mixed juices over the years, preferring to just make something I like myself. The one juice I wanted to duplicate I can, because the vendor sells the actual flavoring he used for that pre-mixed juice. The difference with my version is it is less flavor-concentrated than his, and higher nic.
DIY is a somewhat big initial cost, to get all the stuff for it, but then it is way cheaper. I would guess my monthly DIY costs, if I am not buying a bunch of new flavors, or a new set of syringes, or nicotine, just thinking about juice I go through, is probably less than $10 a month. If you take your $120 and invest it into nic, equipment, flavors, VG, PG, you will probably be able to get a fair amount of stuff, certainly a lot more than a month's worth, and even have some left over.