function consulta1() {

var elemento = document.getElementById('consT');

var checado = document.getElementById('checkBA').checked;

elemento.value = checado ? "R$ 75,00" : "";

elemento.readOnly = true;

elemento.disabled = checado;

}

function consulta2() {

var elemento = document.getElementById('interT');

var checado = document.getElementById('checkBB').checked;

elemento.value = checado ? "R$ 510,00" : "";

elemento.readOnly = true;

elemento.disabled = checado;

}

function consulta3() {

var elemento = document.getElementById('examT');

var checado = document.getElementById('checkBC').checked;

elemento.value = checado ? "R$ 150,00" : "";

elemento.readOnly = true;

elemento.disabled = checado;

}

<form><fieldset><legend><font color="darkblue">Serviços</font></legend><br>

<table width="80%" border="0" style= "border-color: Gainsboro" cellpadding="10">

<tr>

<td>

<input type="checkbox" name="servico1" id="checkBA" onclick='consulta1()'; >

Consulta <td><input type="text" id="consT" size="20" maxlength="35"readonly/></td>

</td>

</tr>

<tr>

<td>

<input type="checkbox" name="servico2" id="checkBB" onclick='consulta2()'; />

Internação <td><input type="text" id="interT" size="20" maxlength="35" readonly/></td>

</td>

</tr>

<tr>

<td>

<input type="checkbox" name="servico3" id="checkBC" onclick='consulta3()'; />

Exames Laboratoriais <td><input type="text" id="examT" size="20" maxlength="35" readonly /></td>

</td>

</tr>

</table><br>

</fieldset> </form>

<!--https://pt.stackoverflow.com/q/258381/101