Sass.scss

@function add-grid($grid-definition) {
  $grid-split: split_string($grid-definition, 'at');
  
  $number: to-number(nth($grid-split, 2));
  @debug $number;
  @debug type-of($number);
 
  @return $number;
}
 
#foo {
  $width: add-grid("5 at 500px");
  
  @if ($width > 500px) {
    width: 100%;
  }
  @else {
    width: 80%;
  }
}

sassy-string.rb

module Sass::Script::Functions
  def split_string(string, key)
    items = string.value.split(/\s+#{Regexp.escape(key.value)}\s+/)
      if items.count == 1
        Sass::Script::Bool.new(false)
      else
        Sass::Script::List.new(items.map{|i| Sass::Script::String.new(i)}, :comma)
      end
  end
  def to_number(string)
    result = Sass::Script::Parser.parse(string.value, string.line || 0, 0)
    case result
    when Sass::Script::Number
      result
    else
      raise Sass::SyntaxError, "#{string.to_sass} is not a number"
    end
  end
end