I've never made one before, but the same principles apply.
Take into account how much a foot of paracord cost me then multiply that by how many feet was used to make the sling (that's my cost), then how much I paid for the hardware. Again that's my cost for materials.
Then factor in your time to make one, now you have your (as I like to call it) base price.
Once you have your base price now you can add a percent like 50%-500% to get your selling price.
Check online to see what other comparable products are selling for and adjust your price from there.
IMPORTANT: Leave your self enough margin so that if some talks you down on a product you know exactly how low you can go and still make a nice profit.