I personally have over 200 gold at the moment. I have not spent a dime on the game. I have spent around 150 gold on Warbringer Axe, Quantum Jumper and Coulson's Revenge, and I'm going to reforge the weapons at some point. I'm level 163 currently. I have spent plenty of gold on CPs, especially in SO2. I've also occasionally bought Unstable ISO-8 with gold if I needed to win a battle or do research.
If I had been smarter in the beginning with gold, I'd probably have 100 more.
A tutorial on how to get gold (some of this MAY only apply in the US):
The first ways to get gold are as above. Level up. 4 star missions. Do decently in PVP tourneys (always enter them NO MATTER WHAT for the 100k silver guarnateed - that's enough to train a level 9 hero to level 10). Always look at the free stuff links, too, as PD will put some gold out there sometimes.
There are two main ways I've gotten to the point where I am: Surveys and AppDog.
The Earn Gold link holds the key to a lot. Every day you are able to take a slew of surveys to get gold. They can be a bit time consuming, yes, but you have the potential to earn 20-30 goldper day. Will you? Likely not, as you have to qualify for the surveys first. I usually will do one and stop, and it can be frustrating not qualifying for survey after survey. However, if you can get the 9 gold survey done once per week, and maybe one or two others, that's huge. You won't get 100 gold per week, but getting 15-20 is not out of the question at all.
If you go to Free Apps on the Earn Gold link, you will see AppDog for both iPhone and Android. In my opinion, this is the best way to earn gold. Why? It doesn't take much time at all. All you do is download free apps, go into them for a minute, and earn their currency (bones). One bone is equal to .1 Facebook credits. When you have 100 bones, that's equal to 4 gold, and you can easily cash them in to get the gold. Recently, I started savnig up for 100 credits, because you get bonus gold when you get to $10 worth of credits. I've cashed that in twice already (and have over 950 bones at the moment), and cashed in my other credits numerous times. I figure I've gotten over 300 gold from AppDog alone, and possibly more.
There is also points2shop where you can again do surveys or plenty of other things, but the currency is similar to AppDog where 100 points is equal to 4 gold. However, you have to save up to at least 1000 points to get a Facebook gift card. Again, no money required. They also have plenty of other rewards for gamers (XBox live gift cards, etc.), so it's not just for Facebook credits.
NEVER use gold for things like Shield Points, buying silver, or staffing planes. ONLY skip Spec Ops missions if it's going to cost you more gold not to (e.g. going for the Epic Boss where you can skip for 25 and you need to spend 30 on a hero), or if you're running low on time and you have a huge research project.
Update: This post was written and posted on the wiki under Gold on January 29, 2013. As of July 8, 2013, I'm now level 265 and have over 250 gold. I've taken advantage of the gold sale twice, and it's mostly been due to points2shop. It's very simple to earn money there, but it's likely best for people in the US, UK or Canada.
I have spent gold on: Lockboxes (Magnetic and Unstoppable. Necessary evil, but I think I spent over 100 total), armory updates (120 a pop) and I did a one-time no-no in that I sped up training for Thundra when I needed her for a deploy in a Spec Ops. I don't think I've bought any new weapons. I have over $20 at p2s right now that I'm planning on getting to $50 by the next sale.