As much as you like herb, there's pretty much no way you'll be able to take just eng and get all your bars from the AH. You can sure try, and if you were really pushing for quote, I'd say upwards of 750-1000 gold. Give the going rate for mithril alone.
Personally I'd drop the herb and go mining/eng. If you're lvl 40, money shouldn't really be a problem. Yet blowing it all on just Bars would be.
Good luck either way though : )