If I have blues, is it more efficient to cash them in for gold, or to salvage them and then craft something new? It seems like I just get more blues by crafting, which are generally not worth the hundreds of gold it costs to make them, but is that true for all crafting?
In general, I'd suggest cashing them in for gold. The auction house is a far, far, better place to spend your money.
If you break down items, you're essentially saying that the components are worth more than the sale value of the item - in most cases, this is not true. Purchasing components at the auction house is frequently a cheaper alternative. Some very low level magical items are worth more as crafting components, but the difference is so slight it's not worth the additional effort.
You then have to craft items at the blacksmith. Unless you are targeting a specific Legendary item, you're gambling with your money and your components. Chances are you're going to end up with a lot of "garbage" magical items with worthless enchantments as a result. This costs you both money and components. When you add this to the money you essentially "lost" by not selling the item in the first place, you're likely to be far into the red on a typical blacksmith transaction.
By contrast, going to the auction house, you can filter the list for rares with low buyout prices, and get just the enchantments you want, for a fraction of the cost of crafting it yourself. If you sell your magic items (and poor quality rares) to the vendors, and spend that money on the auction house instead, I guarantee you will get a much larger bang for your buck - at least until you get into the higher difficulties.
You will generally not make gold crafting until the very highest levels, when you'll be crafting end game loot. The RNG will definitely effect the profitability of crafting, but given the right affixes I'm sure some of the crafted items will sell for a boatload.
That said, it'll be hard to say until the economy settles down a bit and more is known about end game crafting, but low level crafting should not really be done for profit.
The best thing to do with your rares and magic items before the end game is to either vendor them or auction them if you think they'll sell.
Exquisite essence can sell for 1800, so a blie item that's worth 200 gold can turn into 1800, and if it's low lvl you can craft items with monster experience and those can sell up to 40k - 150k and one sale of that will easily cover all the gold you spend crafting the items so yes you can easily make money. Getting monster experience is very common too.