JSFiddle - React, Tailwind, and code Playground

HTML

<select id="dressType">
<option value="0">Dress 1</option>
<option value="1">Dress 2</option>
<option value="2">Dress 3</option>
</select>
<label for="dressDays" size="3"> for # of days:</label>
<input type="number" value="30" id="dressDays" name="dressDays">
<button id="calcBtn">Calculate</button>
<p>Price: <input type="text" size="6" id="priceValue" value="0" readonly></p>

JavaScript

document.getElementById('calcBtn').onclick = calculate;

function calculate( evt ) {

	// get selected dress
	var dress = document.getElementById('dressType').value; 
  var days = document.getElementById('dressDays').value; 
  var price = 0;
  
  switch ( dress ) {
  	case "0":
    	price = 60 + ( 15 * days );
      break;
    case "1":
    	price = 90 + ( 30 * days );
      break;
    case "2":
      price = 120 + ( 40 * days );
      break;
    default:
      price = 0;
  }
  
  document.getElementById('priceValue').value = price.toString();
  
  console.log( 'Days, Dress, Price', days, dress, price );

}