I can see a major collapse of the dbones. I'm going to predict that very soon they could fall as low as 2K. In the longer term, even further than 2k. Prayer is among the abilities that is heavily dependent on the OSRS GP latest updates, when it comes to pricing increasing. For instance, when Piety is released, prices skyrocket with the increase of 1000 gold within a short time.
Why? There's not that many bones in circulation. Everybody needs them, and the supply is drained and prices skyrocket to adjust. Unless there's no updates released which make high-prayer a necessity, it will slow down. When you begin to see new content on prayer that demands a higher prayer level (70-99) It will rise once more.
But if there are no new update, it'll just steadily decline. It might not be a huge amount each week, but it'll decrease a little each month as time passes. It's a skill that is volatile which is similar to herblore. It's true, with frost dragon bots playing to play ball, and blue/green dragons filled with bots I can see the cost decreasing rapidly.
You might say prayer is the most profitable way to earn cash... literally sure, it's. If you're able to like 95 prayers and experience chaos then you get pk and master it and then start making more money then before. This means that prayer will earn you money indirectly. Price will fluctuate up and down daily, but it will only go down over the long term.
Runecrafting: Probably will be the longest duration, generally speaking, you'll need to ZMI If you have access to the lunar spellbook. Alternately, use the Abyss. Penguins are a must and you can earn penguin points on this. Remember to keep in mind tears of Guthix as this xp certainly be useful and fast. An alternative option would be to either make use of the Abyss that is self-explanatory or create earth runes using these FoG gloves as well as Digsite pendants.
Farming is simple and it can be learned as you develop other abilities. Plant Willow (or Maple) and Curry (or similar fruit) in every patch of trees Also, plant a calquat when you are able to. Pay the farmers, and then repeat the planting each day, at minimum in the cheap RS gold case of fruit and maybe more often for the standard trees as they will grow faster. Additionally, you'll want to apply allotment patches and herbs. Remember xp over profit so, for example, cadantine toadflax.