For flowers I look at what they cost to buy from a wholesaler, including the shipping fees. Have you priced them out? That's the cheapest your client can get buy them. So I use that as my base price, then I add my material costs. Then I estimate how much time it will take to make them, x's the amount of money I need to make per hour. If you have other expense you need to add them too.
Always ask enough so you don't have regrets.