[jQuery] getting xhtml custom data in select box

[introvert]

Hello,

I have the following selectbox xhtml structure:

<select id="price_item_1">
         <option value="50" data:interval="1">Volvo</option>
         <option value="100" data:interval="0">Saab</option>
         <option value="49.99" data:interval="0">Mercedes</option>
        <option value="19.99" data:interval="1">Audi</option>
</select>

Now I want to get the data:interval attribute value of the selected
option.
Which syntax should I use with jquery selector?

Currently I have:

$("select[id^=price_item_]").attr("data:interval")

but this obviously wont work.

Any help is highly appreciated!