Around $100 per month. There are some POS systems that cost more, some that cost less, but for the features you need expect to pay around $100 per month.
Also it might not be what you want to hear, but we strongly advise against any free or open source POS for your jewelry store, as the headaches they can create are very rarely worth the cost savings.
However, the only exception is when POS software is completely free WHEN you switch to the payment processor of the provider. Some POS providers are able to offer this as they make more money processing payments than they would from the monthly fee.
We encourage you to try and find out as much as you can about your potential POS system before signing a contract. If you do not have the time to do so, we recommend trying our jewelry store POS finder, we will ask you a few questions about your store and then match you with the right providers for free.