The Great Houses back their currencies on a range of reserves. Technically most are on the Germanium standard. But many also balance out that portfolio with various precious metals (such as gold) as well as other commodities. Some odder items also make the cut in some realms, like art or water (considered precious on desert planets).
For a source, I've consulted the various House Handbooks in their respective sections on their economies and currencies to write the above. Each is different, but there are some commonalities between them.
The realms all still do produce paper currencies and coins minted with various metals, gold being one of metals commonly used.
The value of currency aside, the value of gold as a commodity is going to be based more on planetary economics than the economics of the realm. What is the local supply/demand of gold like? Are there gold mines on that planet? It probably still does have industrial uses, but I'd say no more than what you see in the real world today.
The problem isn't the physical properties of gold. The problem is that since people perceive it to have value, it's pretty expensive, and so using it as a component in industrial uses jacks up the price of producing that item. So whenever possible, businesses use less expensive metals in their products. In the real world tiny amounts of gold are used in electronics, but if the manufacturer can find an alternative, they'd prefer to use that alternative.
The weight of gold is also prohibitive. Imagine trying to move tons of gold between planets on dropships. The heavier the object, the more dropship fuel you need, the greater the expense to transport. You probably need a lot of dropship fuel to carry even a relatively small amount (in tonnage) of gold.
So it has value, but probably not as much in the industrial (military or civilian) applications (who may need small amounts of it, unless they see a deal and want to stockpile the stuff). Gold of decent quality is more likely to be scooped up by jewelers, and perhaps the government itself, for use in minting coins.
The best bang for your buck is probably to find a company that makes jewelry. Assuming we are talking about gold in an unrefined or refined form (i.e. gold bars).
As for its actual value in C-Bills or something, no clue. I'm not sure if there's a chart out there that covers this. The closest I can find is the currencies. The House Handbooks show the various coinnage, composition and value. So for example 1 Marik Eagle coin, made of gold, is roughly 1-1 with the C-Bill (as of Handbook: House Marik set around 3066). The trouble is I don't know how much gold is in a M-Bill. I don't know the size of the coin, or if it's pure gold or mixed with other things (odds are, it's mixed with other things, 100% pure gold coins are historically rare ever since humans learned how to mix metals together to create coins and alloys). The coin minting process also raises the value, so the equal weight of refined gold in bar form is probably worth at least a little less than that M-Bill coin.
We also know that the more advanced worlds of the Inner Sphere tend to use digital payment systems. Physical paper and coin currencies are still in use, but probably see much more day-to-day use away from the centrally advanced worlds of the Inner Sphere.
So you might actually have the best luck in the periphery, or Inner Sphere worlds closer to the periphery, where advanced digital payment systems are scarcer.
But I really do think the value would drift tremendously from planet to planet. Planet with jewelers and industries of all stripes (so a high demand for raw commodities) and a Great House currency mint on-world? But few/no gold mines/refiners? Probably commands the highest value or price for gold.