I'd presume that online seed retailers, like other retailers, need as much as 100% markup as they can get. For example, if a seed costs them $10, need to get near $20 each for singles or the lowest number sold. But I could be way off, with seeds only costing a $1-2 dollars each if buying 1,000s from a supplier; so markups could be much higher for the larger online sellers.
Keep in mind, markup, the selling price relative to the item's cost, is not the same as profits. A high markup does not assure that you make a profit. These companies have lots of costs to cover; markups need to be at least those normal for similar retailers.