overengineered.js

const id = exp => exp
const promo = exp => exp * 2
const trueCash = exp => exp * 10

const makeTotalExp = upgraders => monsters => monsters
  .reduce((totalExp, monster) => {
    const upgradedExp = upgraders
      .reduce((upgradedExp, upgrade) => upgrade(upgradedExp), monster.exp)

    return totalExp + upgradedExp
  }, 0)

const totalExp = makeTotalExp([id])
const totalExpPromo = makeTotalExp([promo])
const totalExpTrueCash = makeTotalExp([trueCash])
const totalExpTrueCashPromo = makeTotalExp([trueCash, promo])