Most cost effective if you are of the opinion (as most are) that expertise is not optimal beyond 1200ish:
Buy PVE rank 27 barrels for MH and OH. Buy only the boots or gloves that have the right mods/enhancements, and power crystals on the GTN. Just saved yourself 40% on the comms for your MH/OH, with which you can now afford WH relics. If you really want the expertise, get the crystals for 500 reg comms or buy cartel market ones for credits.
Did this on my PT, sitting at around 1260 expertise right now. I had the time and comms from other toons to be in full WH the minute I hit 50, but still makes sense if it's your first toon.
Most crafters will make you the barrels for about 1 mil worth of mats each. Start saving!
it is better to replace the power crystals rather than the barrel. trading power for expertise at a 1:1 ratio is great.