Five pounds at $80 an ounce is $6,400. Even if they're hundred-dollar bills, Niles isn't holding nearly enough cash to make the purchase when he shows the Russian sailors the money at the end of the episode.
In The Perfect Guy (1998), Niles is a great and loyal customer to Robert, the shopkeeper. That episode ended with Robert having problems with Daphne and Martin, who were outraged by his prices, and we never get to see Robert or hear about him until this episode. However, when Niles and Frasier ask Robert about the price of the Beluga Caviar, they talk as though they have never met before, or like Niles and Robert never had the history they did.
Before trying it, Roz is unimpressed with the beluga caviar Frasier has her deliver to his house, declaring she had some on a mini bagel once at it was "nothing special." However, in season 9's "Bully for Martin" she gushed about how Roger told her the only thing she needs to bring to their date is her "appetite for caviar and affection." Could be explained away somehow, but a bit inconsistent for what we already know about Roz.