I base my gold spending on Rare cards. In probability, we look at the possible outcomes. You are guaranteed to get at least 1 Rare card from each pack so we can set the probability of that event to 100%. With Epic or Legendary, we would first have to calculate the chance of even getting an Epic or Legendary in our pack and then calculate which packs had the best chance of giving a Legendary I didn't already have. I consider pulling a Legendary or Epic a bonus and not something to use as a basis for decisions. In the same vein, I can't see basing a decision on Common cards because they are too...well, common. If there's a particular one you really need, craft it, its cheap. I had to do that with Spider Tanks. Those suckers would not show up for me. If I ever get a wild hair to play Rogue, I'll have to craft my second Tinker's Sharpsword Oil.
In order to know what you need, you need to know what you've got. I'm using a Collection Manager to track my collection. My personal choice is Hearthhead, but a simple spreadsheet would work equally as well. The core piece of data we want is how many Rares of each set we are currently missing. I don't consider a card 'gotten' until I have both copies. Sure, I could run one and I often do, but most of the time, you want to run cards in pairs. From my collection manager, I see that I'm missing 53 Classic, 22 GvG, and 16 TGT Rares.
Do the Math
Classic has 81 Rares, GvG has 37, and TGT has 36. The core question we need to answer is which pack gives me the greatest chance of pulling a card I don't currently own. Of course, not all cards are equal, some Rares are more powerful than others, but we don't know which card we will get so we treat all Rares equally. We do a simple 'sharp pencil' exercise to answer the question.
Classic has 81 Rares, and I'm missing 53. That means that each Classic pack I open has 53 chances in 81 of being a card I need. We express that as 53/81 or 65.4321% which we can round off to 65%. We repeat this exercise for GvG (22/37) = 59% and TGT (16/36) = 44% .
A Classic pack has a 65% of having a card I need, GvG has a 59% chance and TGT has a 44% chance.
In my particular case, Classic packs are the best use of Gold, but not by a whole lot. You can run the numbers for yourself and see what you stand.