The cost of a string instrument can span an enormous range. Though if you know your budget it isn’t difficult to find yourself a solid set of options! Just be aware that when it comes to instruments, you do get what you pay for. As a violinist myself, I can say that the biggest jump in the quality to price ratio is definitely at the lower end. A $1500 violin is going to sound worlds better than one under $500. Once you reach the upper end of the price range, the difference is still noticeable, but not as dramatic, especially not to an untrained ear.
Further, keep in mind that buying an instrument is an investment. A high-quality instrument appreciates in value over time! This is because the instrument “opens up” as it ages and is played often — think of it like breaking in a pair of nice, genuine leather boots. A good string instrument improves its sound as it’s played consistently and is “broken-in”, thereby making it better in a few years than when it was first purchased.
The following is a set of articles that breaks down instrument choices based on budgets, as well as one that covers outfits, which are basically instrument “kits” that provide you with a case and a bow so that you don’t have to buy those separately.