Security deposits vary depending on the card. They usually start around $200 and go up to $2,500 or more. The higher you want your line of credit to be, the more you will need to deposit initially to act as collateral for the loan.
Your line of credit will usually be equal to your security deposit, but in some cases it might be more or go up after a few months. Some cards will raise the limit on your secured card if you have made all payments on schedule and kept the terms of your cardholder agreement.
Be sure to thoroughly read the terms and conditions of the offer you apply for and contact the issuing bank with specific questions. Whatever the amount of your initial deposit, you should get it all back in the end. You should make sure the deposit you make on your secured credit card is fully refundable.