So far, my custom cards are double the price of my pre-designed cards. That will vary, give or take.
Pre-designed cards are done and done. I can price them knowing each design will sell as is, multiple times.
Custom designs are made from scratch, plus communication, plus (most importantly) it's possible I can't resell the design to anyone else.
By charging more for customized work, you are putting value on your time and talent.
And it can really be a lot of fun! You'll likely enjoy the process, which will inspire you to go the extra mile for the customer. That should give you the confidence to say, "This is how much I charge", because you know how much MORE they'll be getting from you.