To my surprise, they charged me 24 cents per page instead of 12 cents.

Did they charge you 24c per double sided sheet (correct), or 24c per side (wrong)?

Many (other) printing places will slightly discount double sided printing if the job is more than say 100 clicks. Example 12c per sheet for single sided, or 22c per sheet for double sided.

Every time a side of a sheet is printed, a counter in the machine increments by 1 (called a ‘click’). The company that owns the copier – Xerox, Konica Minolta, Ricoh, etc – invoices Officeworks for how many clicks that machine has done in the billing period, at a fixed rate per click. It doesn’t matter if it was two single sided sheets or one double sided sheet, that is still two clicks.

The only saving to you the end buyer is the cost of the paper, which if it was a standard 80gsm bond, is bugger all, unless it’s a big job.

Peter McL writes...

The vast majority of the cost is in ink

Toner makes up a tiny fraction of the cost – and toner, servicing and parts are covered in the fixed click rate charged by the copier owner anyhow. The cost Officeworks et al charges to the customer has to cover a percentage of rent, wages, utilities, insurance, etc etc. and still make a profit.

Lickity writes...

You are paying 12c per 'printed' page, not per piece of paper.

Yes. For example a 50 page PDF is referred to as a 50pp document (50 printed pages). So this will print as 50 single sided sheets or 25 double sided sheets, but in either case it is 50pp / 50 clicks.

If you are ever ringing or emailing for a quote, saying how many pp the document is and if it’s to be printed single or double sided will save any confusion or questions asked. People will often ask how much to print 200 pages double sided, so we have to ask is that 200 sides or 200 sheets because it means different things to different people. 200pp printed double sided means only one thing (100 sheets).