What type of birds make a crested budgie?Is it rare?

http://www.budgerigars.co.uk/specialist/crests/crests.html

 

Here this may help.

I acually have no clue with crested budgies, since it is a type that I don't really fancy. But skimming through that website it seems you need at lest one crested parent to get any crested babies. But also if you know two normal budgie had the gene you would get crested babies.