Unless there's no RuneScape gold updates published that makes high prayer a necessity, it will decline steadily. As soon as you start seeing new prayer content that demands a greater prayer level (70-99), then it'll boost again. Then again, if there aren't any new update, it will just steadily decline. Maybe not a lot every week, but it is going to steadily go down a little each month as years go by. It is only a volatile ability, it is somewhat like herblore.
Well, with frost dragon spiders having themselves a chunk, and blue/green dragons flooded with bots, I will see the price going down RAPIDLY. You may say goodbye is a money sink... directly, yes, it's. Thus prayer does make you cash indirectly. The price will go up and down every day, but will just continue going down in the long term.
Runecrafting: Probably will require the longest time, essentially, you will want to ZMI if you've got access to lunar spellbook. Otherwise use the Abyss. Do penguins and utilize penguin points on this. In addition, remember tears of Guthix because this xp will certainly be fast and useful. An alternative option would be to use the Abyss which is self explanatory or make ground runes with the FoG gloves and Digsite pendants.
Farming is simple and may be trained since you do other abilities. Simply plant Willow (or Maple) and Curry (or comparable fruit) in every tree patch, plant a calquat if you're able to also. Pay all farmers and replicate the planting at least once per day for fruit and perhaps twice for the conventional trees as they grow much faster. Simultaneously you are going to want to use herb and/or allotment patches. Remember xp over gain so, for instance, cadantine p toadflax.
Thieving could be trained through volcano plunder or the theiving guild. Both approaches are quickly and the xp you require will not take long no matter. Since you're almost there for mining, then you probably know what to do. Granite is fantastic xp but tedious while iron could be decent xp and some cash on the side. Alternatively you can do gem rocks which will give relatively great xp OSRS Money with profit and crafting xp.