You also need to take into account the cost of not filling positions. Eventually that cost eclipses the cost of training. Also more experienced developers have

You also need to take into account the cost of not filling positions. Eventually that cost eclipses the cost of training. Also more experienced developers have an easier time jumping ship for their next >20% bump in compensation. Even with senior developers that's more knowledge and training walking out the door. It's also much easier to con a junior developer into staying in one place too long for too little money. You might spend two years skilling them up, but if you pay them well (or even don't) you might get them for 6 years or more. Seen it happen too often.