A case is not just a tote that allows you to carry your instrument from place to place or to store your supplies. It provides protection from the elements and from physical damage. Cases can be flimsy or provide extraordinary protection and can range in price from $80 to well over $1000. Generally, cellists should invest in a hard shell case, as soon as you can afford it, and bass players should select a well-padded soft case with a variety of handles that allow you to support the instrument comfortably while carrying or loading it.
Violinists and violists have a lot more variety available to them and as a result have more decisions to make in selecting a case. Generally, more expensive cases provide more durability, better protection, and more luxurious materials. There are a several criteria you can consider in deciding how much to spend on a case.
First, decide how long you'll have the instrument. If your student is in a fractional-size instrument, you probably purchased it in a package, along with a bow and a case. The chances are good that the case you received will provide good service until your student is ready to move up to the next size instrument. Once your student has moved into a full-sized instrument, you can expect to have the case for a longer period, and you may want to invest in a higher quality case. If you have invested in a reasonably good instrument, it is particularly important to protect your investment from the elements and physical trauma. One manufacturer of high-end cases tests and improves his product by leaving cases outside in the rain, running cars over them, and even setting off explosives near them. Another provides an example of an instrument being left on the roof of a car and having it fall off on an expressway at 70 mph, with no damage to the instrument. You may not subject your instrument to these kinds of conditions and will have to decide what level of risk and price are comfortable for you. However, you should expect to spend several hundred dollars for a case of reasonable quality and durability. Remember that durability and protection are two different things. A case that is solidly built may last a long time but may not provide the best cushioning and protection from trauma. A case that does a good job at the latter may not last long. The trick is to find a case that does well at both.
There are a number of cheaper knock-off cases available that appear as well-made as some of the more expensive models. However, these cases typically don't last long: Straps rip off of covers, bow spinners and hygrometers come loose from the lid, and accessory pocket lids wear and break off from their compartments. In the long run, you will probably spend less money by buying a quality case that will give you many years of service. You'll also have a sturdier product that will provide greater protection as long as you own the case.
In addition to durability and quality of protection, some features you might look for are a “suspension” design for supporting the instrument, weather flaps on the case cover, and a wick (better) or bottle-type hygrometer/humidifier. If you force accessories or music into the case, you risk damaging your instrument, so make sure that there is enough room in the case to store your shoulder rest and other accessories securely. If you must spend much time outside during the winter, consider buying an insulated case cover.
There are several good manufacturers with cases in a variety of price ranges. Among these, in general order of increasing price, are Bobelock, American Case Company, Bam, Weber, and Musafia. You can purchase any of these products on-line, but it is usually wise to inspect cases in person at a violin shop, particularly if you have an odd-shaped instrument that may be hard to fit. A staff member at the shop should be able to provide good advice on the qualities of the various cases they sell.
Finally, keep your case clean and tidy. You can occasionally vacuum the interior to get rid of dust, hair, etc.