What the people at the pet shop haven't told you is that they're about to make a killing off of selling you a mutt for an outrageous price. I'm sure they've sold you on some sort of 'puppy package' and ranted and raved about the quality of their dogs and how you're giving them a good home blah blah blah. You're having the wool pulled over your eyes and you don't even seem to realize it.
You can easily find a great JRT mix from a shelter, rescue, and you can find better information on purchasing/caring for a dog through a local breeder, trainer, or shelter volunteers. When you buy from a pet store, you're pobably buying from a puppy mill. http://www.dogsindepth.com/images/puppy_mill.jpg.
Go to petfinder.com and find an adoptee in your area. JRT mixes are common in shelters because many novice owners buy them expecting that such a small dog wouldn't have such a large amount of energy and possible temperament issues, I'm sure you'll find tons. The advantage of buying from a shelter is that you can usually find one that's already housebroken or at least familiar with basic expectations [potty training, commands etc.], and it is already spayed/neutered and may already be microchipped. Not to mention, since a shelter's goal is to give pets homes, rather than rake in huge profits--the dogs are priced much more reasonably. Don't be duped into thinking that pet store prices can guarantee quality.
With that said, there's no way to predict the behavior of a mutt, only pure breeds are somewhat predictable [when bred well] in temperment, energy, etc. Just because it's mixed with JRT doesn't mean it will act like one. Pure bred JRT's are usually very hyper and energetic and commonly aggressive when not trained well. I would really only suggest a JRT to a former terrier owner, or one who has experience with the breed, as they are very different from what most would expect. The cuteness can lose it's charm quite quickly with a novice owner. If you think that because they're a little dog, they have little energy, or are low maintenance, you couldn't be more incorrect. They are also prone to sight/hearing disabilities/loss as well as joint disease. So be prepared for that possibility as your dog gets older.