Welcome to the forums,
@Weioo!
I think the most cost effective way is medium grind, a good straining bag, and elbow grease. Others have said that GHK's micronized is the most cost effective though you do pay a premium since the production of micronized requires a lot more hands-on than producing medium grind root.
As for extracts, they just don't seem to pack the same punch, but can, as you mentioned, be an effective adjunct to the real thing and extend your kava sensations without needing to drink more grog.
In my experience, there's:
1. Ground root prepared the traditional way.
2. Everything else.
Many people have sworn by the capsules made by Gaia herbs for the initial intro to kava and getting over the reverse tolerance. Take them like vitamins in the evening until you really feel something (you'll know) and then switch to pricier more top-shelf root like the stuff from GHK, Kalm, BKH, KavaBoy, etc. and your money won't be going to waste.
For daytime use, people have said that the Kava Candy from Steve at Ozia is great for on-the-go. I haven't tried it yet, though I will eventually. They're supposed to taste really good. Also, Kalm's concentrate, sold in 10oz bottles and mixed 1:7 with water is fantastic, tasty and effective and is easy to mix up anywhere. Both of these products use kava extract made by supercritical CO2 extraction, so it's a full spectrum extract that's clean of any solvents.