Don't forget there are plenty of f2p items free players can make daily shop runs and makes millions of gp from them too.
Pay attention to new updates. RS3's economy is totally player driven. It is just Economics 101 in such a totally free market player driven efficient market without Jagex artificially manipulating prices of items, prices of items will end up in a random walk. There are always gyrations of prices of most items due to new and ongoing updates.
That said, we are going to have a new quest which we know will impact lots of existing and future new content. Undoubtedly some items will be in demand, including some F2P items too. It is like Necromancy drives up the prices of f2p items like ashes and low level magic items, there may be some new good money making methods for free players too. Keep your eyes peeled.