I think as long as they are biodegradable you are good to go! I have tried to find silk petals for the indoor part since we are going to do bubbles outside that look real, and no luck!!
We asked FTW about this (for our DLR wedding) and it was ok. So I imagine that WDW FTW will be ok with it. They're biodegradable and not harmful to the environment like silks.
I bought muy own basket at Hobby Lobby for $10, and just got the rose petals from disney. One bag of petals is $10.50. I only got one bag due to the size of the basket I bought. Disney mixed white and red petals in the bag. I thought the petals were a reasonable price.
We got small bags of petals for the guests @ $3.50 a pop and a scattering of rose petals in the aisle for $50.
If you need a LOT of petals, www.flyboynaturals.com has freeze-dried petals at good prices. But for what we needed, we only paid a little more to get them from Disney and didn't have to ask someone in our party to put them out.
We did orange rose petals in the Cinderella Coach FG basket and I think it was 55$ like Meg said. It was just enough color foe us so I opted to not do the additional aisle petals.
And we did sachets of orange rose petals for the toss pics and they were 3.50 as well and that was pretty much the extent of my floral.
I was very very happy with how they all turned out. It was just easier for us to use Disney since it was a small amount of floral.