It is not possible to make a textbox part readonly (at least with pure css and html) and part not. However you could do something similar to how twitter bootstrap does it, just change the background color to white to make it appear to be one input.
HTML:
<div class="input-group">
<input type="text" class="form-control">
<span class="input-group-addon">.00</span>
</div>
Override:
.input-group {
width:50%;
}
.input-group .form-control {
border-right:none;
}
.input-group .input-group-addon {
background-color:#fff;
border-left:none;
}
Fiddle:
Bootstrap: