I race karts too and would have to agree, you can spend as little or as much as you want and have a lot of fun.
Just to give you an indication of costs you will need a :
liscence about $180
join a club not sure in nsw but about $250
race suit, boots, gloves, helmet about 1000-1500 depending if you want good stuff or not
a second hand kart with kt100s about $2500 for a decent competitive kart
tyres are about $200 and should last 3 meetings
oil is about $17 a litre and will do 2 meetings
then you need sprockets, plugs, brake pads, chains etc but dont wear that much
a couple of engine rebuils a year about 3-500
You can race for very little, if you are an a real budget get a j, dont wear out tyres or motors but are a little slow, will feel fast to begin with though.
But you can spend lots if you get serious and want to win at big meetings.
Peakrpm what class do you race in? I have a rotax.