レスポンシブグリッドに「同じ高さ」の行セクションを設定するCSSのみのソリューション

.items {
  display: flex;
  flex-wrap: wrap;
  max-width: 1200px;
}

.item {
  display: flex;
  flex-direction: column;
  width: 25%;
  box-sizing: border-box;
  vertical-align: top;
  padding: 0 12px;
  margin: 24px -4px 24px 0;
}

@media (max-width: 600px) {
  .item {
    width: 50%;
  }
}

.item__heading {
  background-color: #d4d0f5;
  padding: 10px;
  text-align: center;
  border: 1px solid #bbbbbb;
}

.item__content {
  flex: 1 1 auto;                   /* IE need 1 1 auto */
  padding: 10px;
  border-left: 1px solid #bbbbbb;
  border-right: 1px solid #bbbbbb;
}

.item__price {
  background-color: #e0f6d9;
  padding: 10px;
  text-align: center;
  border: 1px solid #bbbbbb;
}

<div class="items">

  <div class="item">
    <div class="item__heading">
      Item 1
    </div>
    <div class="item__content">
      Some content that is not that long
    </div>
    <div class="item__price">
      £99.99
    </div>
  </div>

  <div class="item">
    <div class="item__heading">
      Item 2
    </div>
    <div class="item__content">
      Some content that is longer than other items on the same row and sets the height of this section
    </div>
    <div class="item__price">
      £69.99
    </div>
  </div>

  <div class="item">
    <div class="item__heading">
      Item 3
    </div>
    <div class="item__content">
      Some content that is not that long
    </div>
    <div class="item__price">
      £69.99
    </div>
  </div>

  <div class="item">
    <div class="item__heading">
      Item 4
    </div>
    <div class="item__content">
      Some content that is not that long
    </div>
    <div class="item__price">
      £109.99
    </div>
  </div>

  <div class="item">
    <div class="item__heading">
      Item 5
    </div>
    <div class="item__content">
      Some content that is a medium kind of length blah blah
    </div>
    <div class="item__price">
      £29.99
    </div>
  </div>

  <div class="item">
    <div class="item__heading">
      Item 6
    </div>
    <div class="item__content">
      Some content that is not that long
    </div>
    <div class="item__price">
      £99.99
    </div>
  </div>

</div>