I don't understand the cost difference either Sportz but I'll find out. Meanwhile to put to bed whether the Roving Banker Quin exists:

If you go to the Stab racing site: http://www.tab.com.au/Racing/Default.aspx?State=1
and click on a race, then click on bet type Quin, one of the option boxes you can then tick is 'Roving Banker'.

You don't need money in an account to do the above [only if you wanted to process the bet].