Around here you can usually get 15 card boosters from as far back as Ice Age for $3 at a good shop. The only shop that sells for more is the pokefreak downtown who raised his prices to $3.29 for as far back as Mercadian Masques. He sells FALLEN EMPIRES for $2! He had to ask my friend for prices on stuff when he bought the shop. And if you want to trade in cards you must wait a week while he has someone check prices for him.
note: this is the major kid's store, for candy, sportscards ect.
(around 1.5 miles)
At the store farther away, you can get whatever set you want and trade in cards for reasonable amounts of money. This place has a guy who is always there solely for magic.
note: this place is the regional comic shop with a ton of magic: the gathering.
(around 4 miles)
If you don't care about distance, you can go to major shops where there are multiple tournies weekly. Prices are slightly higher, but Bostons PT players hang around there.
note: it's Your Move Games in Somerville
(Around 10 miles from Reading, MA)