Shop around online and see what it would cost you to buy the flower you are thinking of making. Then price out the amount of gumpaste you will need and how long it would take you to make them. For instance, I needed 100 gumpaste roses. They cost me $15 to make and 7 hours of labor....basically $75.00 paying myself $10 an hour. However, pricing out the size roses I needed, they were 1.49 each...or basically double what it would be to pay myself to make the 100 roses. So I chose to make them myself. I charged the client $1 each so I gave myself a teeny bit more of a profit margin for them AND I got pretty good at making them!LOL!
Right now I have a wedding cake coming up in July that requires 100 plumeria. After pricing them out (basically $2 each) I decided to make them instead. $200 to buy them or $75 to make them...I'm making them. The client who is a friend of a friend makes out and I learn how to make another flower. Now, mind you, I actually tried to make a couple to see how hard they were..if they had been out of my skill level then I would have bought them no matter the price.
So it really depends on what you are using them for, if you can make them cheaper than you can buy them, how much profit margin you want to make on them and if they are within your skill level to make.
Hope that helped!
Cat
Not being as talented as cat, nor did I have 7+ hours (and it WOULD take me more than 7 hours. 
), buying them at $1.49 and charging the bride $3.50 each would work for me! I figure that way, I'm actually putting money in my pocket because I can book and sell another $400 cake in that 7+ hours that I would have spent "trying" to make flowers (i.e. "lost opportunity cost"). 
Oh My Gosh...did I stumble upon the one thing you don't do IndyDebi? You, like Rylan, are my CC idols...your cakes are always perfection on a cake stand. I do have to admit, making flowers is something I find fun and I know not everyone else out there will find it to be such a joy as I do...I think being an artist in other mediums propells me to do challenges like that! LOL! BUT, I do know that if I ever come across a flower I can't do...and there are plenty of them out there I'm terrified to even try to make...I will be buying them! And yes, I forgot to tell her that she needs to double the price for what she pays for them if she buys them. Good call!
Thanx!
Cat
I think another thing to take into consideration is the look of the flower....I tried to find nice poinsettias for a cake in December, and there were NONE that looked nice, IMO that you could buy already done. So, I made them.
I also like to be able to "brag" that I made the item myself sometimes (NOT always!)
You have to decide what your time is worth - and add that to the price of the flowers, the "artistic" value IMO has to be considered too.
If it is a flower you like the look of already done, I would not hesitate to do what indy stated.
I took a class from Nicholas Lodge on gumpaste flowers. It was amazing and at the end he gave us a price list for what to charge. I had a request for gumpaste flowers this past week and used his prices. They are a LOT more than you think. I charged $7 for a full rose.
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