Depends on what brand you are using. If it is Cellar Pro brand fruit extract then go ahead and used the regular amount to prime. I asked 3 different HBS's about altering the priming sugar and they told me "no", and to go ahead and use the regular 3/4 cup corn sugar, or 1 & 1/2 cups of DME. There are no fermentable sugars in those extracts. But with any other brands im not sure.
I just used 1 & 1/2 bottles of Cellar Pro brand blackberry extract at bottling time. I have had no problems so far.