Why are most juices sold for the same price? Surely, it can't be equally difficult to obtain juice from different fruits, but surprisingly, they usually cost the same at the store. One exception may be cranberry juice, which is sometimes a little more expensive than the other juices of the same brand (for various reasons, I suppose). 

Does the price become the same for most juices of a single brand because of store pricing policy? Do manufacturers set equal price for them? Or is it really almost equally as hard to make the different juices, so the price is basically the same?
In retail stores everything is "priced" to what the targeted consumer will pay. There is little relevance to production cost to final price

The cost of most food goods is not for "growing", but the energy for fertilisers, tractors,  picking, processing, storage, shipping etc. Your finally price is mostly the cost of energy used (directly or indirectly)

For fruit juice you are paying mostly for picking (fuel), processing (electricity for water pumping and cleaning, electricity to running machines), storage (electricity), and shipping (fuel)

Cranberry juice is significantly more expensive to grow and pick, so this does show as a different price

e.g. Apple Juice. Concentrate purchased at US$1,500 to US$2,000 per ton. Reconstituted 1:5 to 1:7. So one litre of apple juice costs US$0.21 at best case. Yet you buy it for US$2+ per litre!