Marketing research has proved that restaurant goers will spend more if the menu shows prices as numerals without dollar signs or forgoes numerals altogether and spells out the prices.
The researchers are assuming that dollar signs serve to promote thrift by reminding restaurant customers that they're spending money.
And spelling out the prices makes doing math in one's head more difficult, thereby discouraging diners from focusing on how much they will be spending.
I know this is the kind of thing that makes many consumers hate advertising. Personally, I find it fascinating. It's tidbits like this that inspired me to study advertising/marketing.