Regarding real costs, you will have rent, labor, phone, insurance and inventory plus displays.
A few things to keep in mind. The malls are open from 6am to 11pm, 7 days a week during the holidays so labor is a big factor. Also, the mall and the phone company usually want their money up-front.
Regardless of what people thing, November is a very SLOW month for retail. People are not buying for Christmas in November, so all of your holiday sales will be in December. In reality, all of your big days will be from Dec 12 through 24.
Regarding gross sales, My worst December was $30,000 and my best was $65,000. It really varies widely depending on your items sale price.
The biggest secret I could give you to be successful during the holidays is to make your item (in your case Jewelry) as "Gift Ready" as possible. Offer a free gift box and maybe some type of free warranty with the jewelry. Have several different "packages" of jewelry for the customer to pick from quickly. Maybe one package for $30, one for $50 and one for $100.
With RMU sales, you only have a few SECONDS to grab the customers attention and convince them to buy, so make the process as simple and as quick as possible.