As adults it is pretty easy. Allardi have white tails and gasters have black tails. As juvis it is sometimes a little more difficult. What I use to ID allardi if there is a doubt, is allardi at whatever age will always have at least some yellow (often just a little upward pointing triangle in adults) at the forward base of their tail before the actual fin starts.
The other thing is that the odds of a chrystogaster showing up mis-IDed in your LFS is pretty slim. They live on a very isolated island and wouldn't be included accidentally with a group of allardi or clarki.
As far as price goes, that is up to you. $45 is a fair price for an allardi. I think gasters would go for between $200 - $500 a piece.