Maaser Sheni, Chapter 4, Mishnah 1. This mishnah deals with the question of the price at which maaser sheni is redeemed, depending on the place of redemption. The Rambam teaches us that ideally one should not transport maaser sheni produce from place to place in order to redeem it elsewhere according to the price there; but if he did so - that is what our mishnah addresses.
"Podehu k'shaar mekomo":
"Hamolich peiros maaser sheni mimkom hayoker limkom hazol, o mimkom hazol limkom hayoker" - the mishnah lists two cases:
From an expensive place to a cheap place - he transported the maaser sheni produce from a place where the produce sells at a high price to a place where it is cheaper.
From a cheap place to an expensive place - he transported it from a place where the produce sells cheaply to a more expensive place.
In both cases the halachah is the same: "podehu k'shaar mekomo" - the redemption is made according to the going rate and value in the place where the redemption is actually carried out. Wherever the redemption takes place, that is the price by which the produce is redeemed.
"Hashevach lasheni v'hahotzaah mibeiso":
"Hameivi peiros min hagoren la'ir, v'kadei yayin min hagas la'ir" - he brought produce from the threshing floor, the place where it grew and was processed, into the city, or he transported jugs of wine from the winepress into the city. At the place of production prices are always lower, and in the city he will end up paying more - so the value of the produce has risen. Ideally he should not have transported it, since that is its place of production.
"Hashevach lasheni" - the added value is considered to belong to the maaser sheni: the redemption will be made according to the higher price, and as a result the maaser sheni money will increase.
"V'hahotzaah mibeiso" - the transport costs and the like come out of the owner's own pocket, and the maaser sheni does not bear responsibility for them.
It turns out that when he redeems the maaser sheni, he redeems it for a higher sum, and he will have more maaser sheni money to spend in Jerusalem.
In summary: in this mishnah we learned that one who redeems maaser sheni produce redeems it according to the rate of the place where the redemption is carried out, whether he transported it from an expensive place to a cheap place or from a cheap place to an expensive place. And when he brought produce from the threshing floor, or jugs of wine from the winepress, into the city, and their value rose - the increase goes into the maaser sheni money, while the transport costs fall upon the owner and come out of his own pocket.