Having done my fair share of bulk trading, I see two specific problems with holding to a fixed rate of 10i/action:
1. Not all actions are equal in length.
Levelling to collect dirt (regardless of shovel ql or enchantment) always takes 6sec/1dirt, and stamina drain has no effect on timer length. With a high-ql high-woa shovel and decent digging skill and body characteristics, you can get the timer down to 3-ish sec per digging action (the same holds true for mining). It's not hard then to see why there's oversupply of dirt, shards, and metal lumps on the market.
Other actions have longer timers, are affected by stamina drain, and cannot be sped up by increasing tool quality or by woa enchantments because they use 'hand' or no tool at all (combining arrow shafts and arrow heads for example).
1. Not all actions are equal in difficulty.
Some actions require much higher skill to reach a decent success rate (making mortar, arrowheads, and arrows are some that I have experience with).
In real-life free-market economies these translate into the following observations:
1. Menial workers aren't paid for the number of actions they perform, but for their time.
Whether they're working in a factory that makes boxes or one that makes cars, where the time required to make each unit is vastly different, they'll be paid a comparable hourly wage.
2. Skilled workers (insofar as their skills are in demand) are paid more than unskilled ones.
Skilled workers can either do things unskilled workers can't, and/or they are more productive and are less likely to waste materials.
The relatively low price for dirt and high price for mortar, in terms of the 10i/action rate, are natural consequences of these principles in a free market. As others have alluded, the Wurm economy can never be like a real one for several reasons. One major reason is the fact that toons live forever and skills are never forgotten. This, coupled with the fact that new players are always the minority, means that the traditional pyramid distribution is turned upside down in Wurm: there's a huge number of high-skilled toons relative to low-skilled ones; and a huge number of producers relative to consumers.
It's hard to see a way around this. Most changes to correct the oversupply that is driving down prices would make the game less fun or drive away established players:
Make crop/hunting/fishing yields much lower. In real pre-industrial societies, over 90% of the population are farmers. In Wurm, 1s worth of food can feed a toon virtually forever and 1s can be found in your first day of foraging.
Make it impossible to store food and many other goods indefinitely in FSBs and BSBs. Almost everything in real life rots after time and needs replacement. Not so in Wurm, where vegetables grown 50 years ago are still perfectly fresh if stored in an FSB. Realistic rates of decay (faster for veg, slower for grains and some processed foods like flour) would stimulate demand. Same if planks were to moulder and nails were to rust.
Make skills subject to decay if they're not used (perhaps with a 3x regain rate, like after death). In real life, we forget much of what we don't use. People really can't know it all and do it all, at least not at any given moment. Market forces slowly draw people into high-demand trades and out of low-demand ones. But switching trades comes at a cost of forgetting some of what you've learned, so the decision isn't taken lightly.
Make toons grow older and die (permanently) of old age. Some characteristics slowly drop after a certain age, such as body strength, and at some point the toon is simply no more. This creates a reason for new low-skilled toons to be 'born' even with a stable player base and helps maintain the natural pyramid shape of a healthy economy.
I can't see any of these changes being popular and I'm not sure if I'd enjoy Wurm myself if they were implemented. But without them, it seems inevitable that prices for goods will continue to drop. Prices aren't even corrected when fed-up players quit – high-skilled toons, non-decaying bulk items, and high-ql tools etc. are simply passed on to other players and the general oversupply of everything – from skills, to bulks, rares, and enchantments – continues.