This discussion is mostly based upon d20 rules, specifically the d20 D&D rules version 3.5 published by Wizards of the Coast. Most other magic systems have the same effect, but see the assumptions.
One thing that does not seem to be done when constructing game worlds is to apply magic, and the economics of magic to the overall society. There are a number of D&D spells and magic items that should have a significant impact on the overall economy of a world. Another area that is not considered, and is related to the ecomics of magic is the use of magic as a substitute for technology.
In a standard D&D world, the number of spell casters is dependent upon demographics. Specifically it is dependent upon the size of the population center, with high level spell casters only normally found in large cities. Historically, there have been many large cities in the ancient with populations of over 25,000 (that being the upper limit on the d20 Dungeon Master Guide tables). The Roman Empire, ancient China and Egypt had plenty of such large cities although most of the land was populated by small vilages. If you move north though in the same period, all you find is small thorps and hamlets to use d20 terms. The average population center in the trans-Alpine region was 100 people. It is up to the DM to decide wether her world looks like the Roman Empire or the forests of Germany.
The number of magic items is dependent upon the number and level of spell casters and how long there have been spell casters. The D&D rules have no mention of magic items wearing out, decaying or otherwise needing to be replaced unless they are purposefully destroyed. For example magic swords might break in combat, but the bed of healing won't lose it's magic or fall apart no matter how old it is.
issue of apparently unlimited energy if using magic. perpetual motion issue. possible solutions:
Cost: 110 GP for minimal caster level; Duration: permanent; This is a replacement for torches, oil lamps and candles. Casting of the spell on a object provides light equivilent to a torch, roughly a 20 foot radius of light.
Disadvantages: This is not a cheap spell with regards to replacement of mundane light sources. Torchs and such cost so little that it would take 30 or more years to recover the direct cost. It is also subject to being dispelled.
Advantages: The spell is essentially permanent.
Cost: to expensive for normal feeding of a city or even small town. useful maybe as emergency ration scrolls or wand.
makes the creation of canals easy, albeit not cheap. the work could be done with hand labor, maybe even cheaper and just as fast.
eliminates miners, but does not eliminate blacksmiths, etc. still has to be forged into useful items
in lieu of road work by wizard. also quick and relatively cheap city walls.
cost: 24,000 gp no apparent limits on use per day. useful for long range travel. won't eliminate merchant ships or carvans. cities with multiple gates can charge money for short cuts between cities. think of the Manticorian wormhole or Suez Canal. disease vector. won't create a mage class per se, but could create mage merchants. if used once every 5 minutes, 24 hours a day, 365 days a year will pay for itself in a year (not counting manning or costs of the buildings to control access). no sieges of cities unless teleport can be blocked. always have a way for food and weapons to get in and excess or wounded people to get out. need to find range of teleport blocking spells / items
can be used to move troops from relatively central barracks in say capital region to outposts as needed. maintain smaller overall army since reinforcements can move to any one border quickly.