My Mercere magus is considering adding (which will probably mean creating) an ice trade to the covenant sources of income.
Of course, there really was not an ice trade in medieval Western Europe. I have no idea what to estimate a reasonable price to be. Initially, I suspect the answer to be "What you can get," and the mundanes might initially not see much of a use for ice, even in a hot summer, so that will take some marketing. If an ice trade can be established, what in Mercury's name would be a base price? Penny on the pound-pint? Shilling on the pound?
The cover for mundane purposes involves lining a closed wagon with cork, and selling the ice in wheel blocks, like cheese. We say we freeze in winter, keep it in a lined cellar, and sell in summer. You want ice in late August? I'm sorry - we have sold our stock. We will be sure to increase our freezing pans this winter.
I'd say "Greater" if you reserve it for what it was being used for during the actual middle ages (ie, cooling the drinks of royalty during summer). However, if you have a room that can actually MAKE the stuff, economies of scale suggest that it will quickly become "typical" to "poor".
Some articles I found on it: (likely similar to what you found):
The wiki entry on Ice houses - looks like the English started doing it in the 1660's, although other folks did it a lot earlier (Persians and Chinese were doing it in the 400 BC era) - en.wikipedia.org/wiki/Ice_house_(building
The Ice Trade - ah, here we go. Yep, looks like, as an Industry, it really only took off in the 1800's. Prior to that, wealthy Europeans were doing it in the 1600's, but not a whole lot of formal stuff prior to that. en.wikipedia.org/wiki/Ice_trade
I would say that medieval Europeans would know that you could use it to preserve food - simply because you can do that during the winter. However, it is also known to be able to cool the drinks of royalty, which is probably the big seller.
The issue seems to be how much ice you would make. If you only make a few tons a season (not all that much, honestly ), then you'd probably charge up the wazoo for it, and sell it as a luxury item. but if you expand production by a couple of orders of magnitude, peasants could start using it to store food - which means you'll start competing with the salt trade. And because everyone needed salt (for preservatives), everyone taxed it, which is why salt was so freakin' expensive.
However, as a preservative, salt probably works better (ie, it doesn't melt). So really you'd need a LOT of ice to keep the freshness up. So it would probably still only be used for luxury foods - albeit on a slightly broader scale.
EDIT - and you'd probably need to develop your own infrastructure to STORE the ice near your patron's estates - or they'd need to build their own ice houses, as their decedents will do in the 1600's.
The ice trade was well developed in Roman times; and drinks cooled with ice/snow where very popular not just with the nobility, but with the commoners too. You could buy them at thermopilia, basically street "bars" that sold you cold and hot food and drink, to eat at the "bar" or to carry away. Just to give an idea of how large scale it was, more than 150 thermopilia were found in Pompeii, if I recall correctly. At least in Italy, using ice-houses to preserve food remained common in the middle ages at least in the (neighbourhood of) the Alps, and certainly in Sicily too; again, not for the nobility, but for commoners too. So, nothing too exotic.
I figure that we will have to pour resources into it for a few years to develop the income source from Poor up to Typical or more, even if we're selling (ultimately) to royalty. There are wagons and ice houses to build, distributors to invest in, samples, officials to "induce", and so on.