As the comment suggest you you should try like this below code is working as you expected.

     <template>
      <form>
    <div class="row">
      <div class="mb-3 col-sm-4 col-xs-12">
        <h6 class="text-dark" for="purchasePrice">x:</h6>
        <input
          type="text"
          class="form-control"
          autocomplete="off"
          v-model="purchasePrice"
          @keypress="isNumber($event)"
        />
      </div>
      <div class="mb-3 col-sm-4 col-xs-12">
        <h6 class="text-dark" for="salePrice">y:</h6>
        <input
          type="text"
          class="form-control"
          autocomplete="off"
          v-model="salePrice"
          @keypress="isNumber($event)"
        />
      </div>
      <div class="mb-3 col-sm-4 col-xs-12">
        <h6 class="text-dark" for="marginResult">z:</h6>
        <input
          type="text"
          class="form-control"
          autocomplete="off"
          disabled
        v-model="marginResult"
        />
      </div>
    </div>
  </form>

<script>
 export default {
  data() {
  return {
  purchasePrice: '',
  salePrice: '',

   }
  },

  methods: {
    isNumber(e) {
    if (e.keyCode < 48 || e.keyCode > 57) {
        e.preventDefault();
    }
    }
    },

computed: {
    marginResult() {
        return (this.salePrice - this.purchasePrice).toFixed(2);
    }
},

}
</script>

Screenshot for you to check if code works or not

opp's your are expecting blank it returns 0.00 but you get the idead.