Vue

HTML

<div id="app">
USD:<input type="number" v-model="priceInUSD" />
RUB: <input type="number" v-model="priceInRub" />
</div>

Vue

new Vue({
  el: "#app",
  data: {
	statePriceInUSD:0,
   statePriceInRub: 0,
  rate: 65.395402,
  },
  computed: {
priceInUSD: {
  get(){
    return this.statePriceInUSD;
  },

  set(newValue){
  			console.log('USD new value: ' + newValue);
        this.statePriceInUSD = newValue;
        this.statePriceInRub = newValue * this.rate;
  }
},

priceInRub: {
  get(){
    return this.statePriceInRub;
  },

  set(newValue){
  		console.log('RUB new value: ' + newValue);
      this.statePriceInRub = newValue;
      this.statePriceInUSD = +newValue / this.rate;
  }
},
}
})