You can put the value inside a bullet like "span", and give it an ID.
<div id="precificacao">
<h3>R$ <span id="valor">870,00</span></h3>
</div>
Then you can use the JS function to change the SPAN value when a checkbox is selected:
<script type="text/javascript">
function troca1(){
var valor = document.getElementById('valor');
valor.innerHTML = '870,00';
};
function troca2(){
var valor = document.getElementById('valor');
valor.innerHTML = '970,00';
};
</script>
You call a function with "onclick" in your checkboxes.
<form>
<input type="radio" name="cb1" onclick="troca1()">
<input type="radio" name="cb1" onclick="troca2()">
</form>