do not start with being concerned with her selling cost. First figure the cost of your materials (is there a way to lower these costs such as buying online or a garment district in a major downtown area). Then figure your time. Can you production line a few at a time? At that point you can then figure a cost for the production. When making small items production line (whether it is fabric items, wood product, jewelry etc), you can either do add a 2 - 2 1/2 x's your materials cost or you can figure in an hourly wage. Example (these numbers are just pulled out of the air, don't hold them as actuals)- a mini quilt cost $15 in wholesale materials cost. You can make 3 at a time and it takes you 6 hours to make them. You could charge either $15 plus $30-$40 for each = $45 - $55 per. Or you decide your time is worth $15 per hour. The you would charge $15 plus 2 hours work (3 made in 6 hours) $30, so the cost would be $45. I would calculate both ways and this would give me room to negotiate with the store owner. BUT THE MOST CRITICAL IS TO GET THE AGREEMENT IN WRITING! Even if it is your best friend. Contracts save friendships. I have had a retail store for almost 30 years and have worked with many newbies. And I sell a lot of my work both at the store and online.. Good luck and keep it fun.