Jester David
Hero
Three carts is probably all you could manage (one to the left, one to the right, and one straight ahead) for 550gp including a return trip. On your forth trip your break even.50gp only buys 500lbs of wheat if wheat is available. Same goes for sheep and cows. If you can open the portal and bring 2500lbs of wheat through (ie - a draft horse pulling a cart), then sell it for twice the listed rate (ie - in the middle of a desert), then you've just made 200gp. We'll call it 150gp on the assumption that you need to get back again.
And why would you bring through a single cart? 6 seconds and a 10 foot radius should be able to get you multiple cartloads. And you should be able to haul something worthwhile back again - even if it's just sand.
Those ships you chartered cost 10,000-30,000gp each. Why would you ever build them when you can throw down 18,000 and get instant completely secure travel?
Assuming there are enough people in the desert to need 7500 lbs of wheat.
As for ships, well the 18k figure was the minimum assuming the wizard was working for free. Assuming they were doing so for money could easily double the cost. Buying that 30k galley could be 6000gp cheaper and it can carry 150 tones per trip. Also known as 30,000gp of wheat paying for itself in a single trip.