It is supply and demand, like anything else
Gold has a price because producing it takes time, and people who would rather not spend that time are willing to pay for it. Anything that changes how much is being produced, or how badly people want it, shows up in the rate — usually within days.
That is worth saying because rates are often treated as arbitrary, or as something sellers set together. They are neither. A seller who prices well above the market does not sell, and one who prices far below it is either clearing stock or is not holding what they claim.
What pushes rates down
More supply, essentially. A method that produces gold faster than the ones before it puts more into circulation, and quiet stretches in the game's calendar — no major update, nothing new to save for — leave demand flat while production continues.
The effect is gradual rather than sudden. Supply-side changes tend to show up as a slow drift rather than a step, which is why rates can look stable week to week and noticeably different over a couple of months.
What pushes rates up
New content is the clearest driver. An update that introduces something expensive and desirable creates a lot of demand at once, and the rate moves before the supply catches up.
Enforcement pushes it up from the supply side instead. A large wave of action against gold farming takes production out of circulation quickly, and prices tend to firm afterwards. Nobody outside Jagex knows the scale of any given wave, so this is a direction rather than a number.
Seasonal events and temporary game modes matter too, mostly by moving attention. When a large part of the player base is occupied elsewhere, both the producing and the buying side thin out at the same time, and the effect is less predictable than the other two.
Bonds as a reference point
Bonds give the game an official route between real money and in-game value, which makes them a useful anchor when you are judging whether a rate is sane. They are not a price ceiling in any strict sense, but a rate wildly out of line with what a bond implies is worth a second look.
We have a separate guide comparing the two properly, because the answer depends on what you are actually trying to buy rather than on the headline rate.
What this means for timing
Mostly, that timing is the wrong thing to optimise. The gap between a good day and a bad one is usually small next to the gap between sellers on the same day, and smaller still next to the risk you are accepting by buying at all.
Comparing sellers is where the real difference is, and it is the comparison you can actually make. Our live price tracker is there so you can see what the underlying market is doing rather than take anyone's word for it, including ours.
The short version
Rates move for visible reasons — supply, updates and enforcement — but on any given day the spread between sellers usually matters more than the day you picked.