With woodworkers, oak logs sell for 0/2/40 and you can harvest them at level 17.
I'm actually treating this as the base income for my character. You can make more money through, say, cedar planks, but that requires selling them on the marketplace or directly to another player, as well as making sure you have a reliable source of blades, etc. Oak logs can be treated money in the bank, and there's always the chance you can sell some for more than their base price or make use of them once you hit higher craft levels if you don't have to cash them in.
I've got four foresters who can all chop oak when I don't need the companion slots for anything else, so it's a pretty nice continuous income as well. They work at 1/3 speed of the hero, so it's nearly always the equivalent of having a second hero working for me without needing to rest.
Another useful trick if you have no other source of blades; fish for speckled whitefish (the brown ones...) in the raised lake in Goldenfog plains. Two of these can be traded for a single copper blade (7 mech strength) at the hunting camp in Riverwood (1 minute, 0.2 energy barter). At Goldenfog you will catch 2-3 ever 15 minutes (3.6 energy) with 100% chance (only 70-75% elsewhere) so can get 4-6 blades every hour that way. Not the most efficient method ever, but if you are really hurting for blades and cash then it's a backup.