Except in the high 1200s, population density was always lower than carrying capacity.  Medieval agriculture was generally highly inefficient for technological, social and cultural reasons.  The economy is going to be driven by:
a) The form of primary land tenure of the peasantry
b) The form of primary land tenure of the nobility
c) The level of social stability since the last social (ie: military) or natural crisis

I strongly doubt the ability to compute prices and times to produce goods from existant medieval wages.  Wage labour was concentrated in abnormal economic entities: monasteries.

Urban subsistence should be viewed primarily through the relationship between urban communities and the local aristocracy.  If cities have free burgage and low taxation then expect the bearing population to be restricted by the burghers themselves in order to concentrate accumulation.  (Also, expect the nobility to raid the town in the next war.)

Logistics mean that towns—ie villages that run a yearly agricultural fair—will be no more than about two weeks walk from each other in settled areas.  Riverine and sea going transport will locate such towns.  Cities, as in "possessing a cathedral" will only really form at the nexus of multiple trade routes OR at the central transport concentration of multiple depths of town based trading networks.

Accumulation in the hands of the burghers is already set: it is primarily in the form of housing, hoards of food (possibly collective, ie town hoards), and petty commodities.  It gets wiped out every 50 years by warfare or civil warfare.

Accumulation in the hands of the church[es] will generally take the form of hoards, periodically used to glorify the structure of their worship, ie: buildings, magnificence.  Churches are a damn good thing to loot.  Expect the bishop equivalent to peregrinate with the loot at any sign of trouble.  I figure this model is the best model to use for accumulation by magic users.

Accumulation in the hands of devotional communities is going to be in the form of productive apparatus, fixed in place.  Manured fields.  Heads of cattle.  Centralised domestic production apparatus (spinning, weaving, dying, brewing, distilling).  Good to loot, but not much of it is saleable.

Accumulation in the hands of the nobility is a mixture of less efficient devotional communities primarily geared towards producing luxury goods and weaponry, combined with the social ostentation of the church.

In any case, if you want something made, or to purchase an item, your best bet is to find someone powerful who already owns such an object and will give it to you as a boon in exchange for fealty or duty; or, to own a person who can make such a thing for you; or, to loot either of the two preceding.  I recommend looting the bourgeoisie—they aren't organised as a community fit for war, and you don't get excommunicated for burning down their productive apparatus.  

If you really must purchase something, then objects will be available at a yearly basis at a fair.  If you feel generous, run a couple of suitably large fairs in distant towns.

I just realised I had better answer the question posed in the first place:

Between 99% and 95% of the population should be directly involved in agriculture. ( Table 5, Malanima, *Pre-modern European economy* http://books.google.com.au/books?id=C1Ej4VEPwSgC&pg=PA248 )  While Malanima's table shows rates up to 15% in cities over 10,000; these centres were in the South, and do not consider urban agricultural occupations.  Remember also, that these urbanisation concentrations will be in the equivalent of London, and that smaller cities and towns will have significant populations whose primary occupation is agricultural.