Before you buy fresh flowers online, check out their "cheap" prices. Many sites insist you buy a large quantity of flowers without really giving you the true wholesale price that a flower shop would be charged.
Standard calla lilies have large flower heads that can range between 4" to 8" long. Miniature calla heads are usually around 3" or less.
The stems are thicker around as well. Keep the size in mind when choosing your flowers. Standards only need three or four stems to make a good sized arm bouquet like the one shown below.
For for information, check out the shopping link above for market price and ordering options.