Saeviomagy said:
For 170 minutes, you can ferry through 1 mule a round, carrying 690 pounds of goods.
Lets trade something normal. Like... salt. 5gp a pound.
That's 3450gp on each mule (you can afford to put them at heavy load, because they're only taking a few steps.
170 minutes * 10 rounds *3450gp of goods = 5865000gp per circle.
Assuming no more than a 1% difference in price from end to end, that's 58650gp per casting of the spell. Your investment is 1700*8cp+1000gp+2cp (cost of drivers, 1 per 10 mules) = 1136.02gp per circle.
That's 586.5 TONS of salt. I honestly can't say how much salt a large city might use in a given period of time, but I'd guess that one could use this much in a fairly short time. I'd say it would have to be a large city since this is almost 6 million gp we're talking about here.
I see three problems with this calculation:
1. It assumes that the wizard casting the spell is working for free. Unless the merchant happens to be a 17th level wizard himself, the wizard's time would cost you most of that 58,650GP. There's not going to be much in the way of price reductions since the number of 17th level wizards is never going to be that high.
2. It doesn't take any sort of volume pricing into account. That 5gp per pound price is the final retail price. That's what it costs Joe Adventurer to buy a few pounds for his expedition to the Barrier Peaks. Merchants trading in bulk will probably be buying and selling for less per pound. The guy hauling it from the mines probably buys it for 1.25 gp per pound and sells it for 2.5gp per pound to a merchant in the city, who turns around and sells it a few pounds at a time for 5gp per pound. That assumes there aren't even more middlemen in there somewhere.
3. It also assumes that you're going to need 586.5 tons of material moved from one spot to another. That does happen, but most of the time that stuff is then going to have to be distributed from there. That means you will just have changed the pattern of hauling stuff, not eliminated the need for it. You'll need to collect everything needed to make a profitable teleport in one spot before hand and distribute it afterward at the other end. You will eliminate some long haul traffic this way, but that's about it.
A couple of other thoughts:
The low number of wizards limits how often you can do this as well as keeping prices high.
I wonder what would happen to the price of diamonds if this spell came into widespread use. You could see material prices going up as it became vital to the economy as well as being a luxury item barring any improvements in the technology of finding them that would increase the supply. That would also tend to lead to wars and intrigues over this new strategic resource.
If you want to talk about items that can cast the spell or permanent circles then yes, as I mentioned, it becomes economical despite a higher initial investment.