The eggs I bought were $4.99. (double)

The eggs I bought were $$eggsprice. (single)
//As one can see, the difference between single and double quotes in this case is that in single quotes, everiything written in between is taken as it is. In double quotes, all the variables set (in this case $eggsprice) will be translated into the values that the user set (in this case 4.99).