Sass.scss

@function add-grid($grid-definition) {
  $grid-split: split_string($grid-definition, ' at ');
  
  $number: nth($grid-split, 2) * 1px;
  @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(key.value)
    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
end