For use with spaceplan

showbest.js

// Paste in console
var getThings = function(){
    var divs = [...document.querySelectorAll("#manufacture__container > div")]
    var things = divs.map(function(e){
        var result = {e};
        var spans = [...e.getElementsByTagName("span")]
        spans.map(function(s){
            var str = s.innerText.replace(/[^/.0-9]/g, '');
            var a = str.split("/");
            var num = +a[0];
            var den = +a[1] || 1;
            result[s.id] = (+num) / (+den)
        })
        
        if(e.id === "item__spudGun" || e.id === "item__potatoLauncher") result.powerGain *= 1000;
        var cur_power = +document.querySelector("#watts--total").innerText.replace(/[^0-9]/g, '');
        result.score = result.cost * (cur_power + result.powerGain)/result.powerGain;
        result.name = e.getElementsByClassName("manufacture__name")[0].innerText;
        return result
    })
    return things
}
var getFilteredThings = function(){
    return getThings().filter(function(e){ return e.cost !== 0 })
}
//console.info(getThings())
//console.info(getFilteredThings())
//console.info(getFilteredThings().map(function(e){return e.score.toExponential()}))

var highlightBestThing = function(){
  var things = getFilteredThings().sort(function(a,b){return a.score - b.score});

  var bestThing = things[0];
  console.info("highlighting " + bestThing.name);
  
  for(var i = 0; i < things.length; i++) {
  	things[i].e.style.color="#fff";
  }
  things[0].e.style.color="#ff0";
}
//console.info(highlightBestThing())

var timer = setInterval(highlightBestThing, 500)

Cleaned up the code and fixed for latest version of Spaceplan

Fixed unaffordable items being ignored. Changed highlight color to yellow.