PaRtiZzaN, Раз у тебя 1Гиг стоит 2р, то к чему там case плодить?
Как то так можно сделать.
<form>
<p><select name="" id="hdd" onchange="fnc()">
<option value="199">HDD 1</option>
<option value="299">HDD 2</option>
</select></p>
<p><select name="" id="cpu" onchange="fnc()">
<option value="0">not marked</option>
<option value="100">CPU 1</option>
<option value="300">CPU 2</option>
</select></p>
<p>
<input type="range" id="rng" min="30" max="500" step="10" value="30" oninput="fnc()">
<span id="space">30</span><span> Gb</span>
</p>
<p><span id="total_sum">259</span> <span>₽</span></p>
</form>function fnc() {
let hdd = parseInt(document.getElementById('hdd').value, 10);
let cpu = parseInt(document.getElementById('cpu').value, 10);
let rng = parseInt(document.getElementById('rng').value, 10);
space.innerHTML = (rng);
rng = rng * 2;
let price = (hdd + cpu + rng);
total_sum.innerHTML = (price);
}