JSFiddle - React, Tailwind, and code Playground

HTML

<p id="ori"></p>
<p id="new"></p>

JavaScript

let queryStr = '{ "averageCost": { "lt": "10000" }, "test": { "gt": "12345"} }';
document.getElementById("ori").innerHTML = queryStr;
const regex = /\b(gt|gte|lt|lte|in)\b/g;
queryStr = queryStr.replace(regex, '$$' + "$1")
document.getElementById("new").innerHTML = queryStr;