Instead of purchasing any 1 thing in particular, how about purchasing things in g.e. with present offers below 5 percent ge value. Makes it tougher for items to crash in value. Creates a unspecified demand. Junk items can regain value as a percentage of 20gp will qualify easier than a thing costing 20,000. Presently the sole market for several items are sets, store price and alch value, this could give value to collections of non value things as it provides resistance against them crashing farther. Since their value can only go up, pretty produces a stock exchange with thousands of items, it's a good investment. The marketplace will purchase and delete and you can always have the cash for selling 5% below out option from game. To make collections issue a dividend.
I guess it depends on what you want the system to achieve. With a gross. You are currently strengthening the thing will increase in the day's close, by around 20 percent in the slightest. 2 years down the trail, with an increase we could observe inflation of commodity rates. Of course this depends on individuals. As the ge regularly updates a few times every day following a set handful of hours.
The day's item ought to update at the exact same time based off data. This could make it difficult if the set price is above market to guess and gain from. I would suggest the item not be shown Until later, maybe even delay. Considering the main source of these kind of things in RuneScape are bot farms / gold farmers it's worth considering how this may interact together. Because they could not oversupply will it make them a load of cash?
I agree alot of careful thinking has to be placed in before actually launching something like this, but I do believe that the overall idea of turning cash into things and ruining it to get a chance of a rare pet could actually work in-game, Jagex just must take it out of here. Simply use the amount to buy whatever thing it's the current moment. If yesterday 10 bil was put then use 10 bil today towards thing x. Do not tell the players that item was chosen until after the money has all been spent, or merely the following day or anything.