Here's what I do and it works for me but may not work for everyone:
I order sample sizes or small sizes when they are on sale - lot's of varieties. Then, when I find a few that I really like I'll order them in the absolute highest MG nicotine possible and ask the vendor to double the flavoring. That way I can dilute it with either PG or VG (though I always use PG) as much as doubling the total volume.
Also, Tasty vapor sells a "doubler" just for this purpose. So if you order from them you can just order the highest mg nicotine possible and some doubler and use the doubler (which is just 0mg e-liquid that's flavored to match the juice you ordered) to dilute your juice down and thereby double it.
I figured out that my e-liquid is currently costing me about $5 a week (and I use 3ml per day) now that I've found a few flavors that I like.
DIY is very easy to do too, but I usually reserve that for the very, very easy flavors because I can't afford to experiment. So I do straight fruit flavors, menthols, chocolate.. and if I have a flavor that I don't like as it is I will mix it with something that I think will help or else drown out the flavor with some strong menthol flavoring.
Here's a good e-juice recipe calculator:
eJuice Recipe Calculator