We originally booked via Tours by Locals, but the driver never showed. So we asked the bell desk (which also arranges tours) what the cost for a driver would be. So for 12hrs incl gas was about 1mln IDR. If we paid in cash (which we did), it was 30% off. But, it was just a driver, no guide or anything nor the entrance fees.
Entrance fees to the temples are not cheap - $18 for one and $20 for the other per person. Guides can be booked on the spot at the temple for 100,000-200,000 per guide, depending on the number of people (we had 5 and one spot was 150,000IDR and the other 200,000IDR). the guides were for an hour and did cover the stuff very well.
So the cost was about 700,000 for the car + 350,000IDR for the guides for 5 people + some 550,000IDR per person for the entrance tickets for the temples + tips.
the sultan palace was cheap - it was like 50,000IDR for the guide and like only $1USD.
We did Sultan palace - prambanam - borobudur. Started at 930AM and got back at 6PM. No lunch stop.
hotel to sultan palace was 30min. sultan palace to prambanam was about a hr. there to borobudur was about hr and half. and from there back to the hotel was about hr and 15min.
The driver did take us to a "spot" to buy tickets, not at the temples, but the prices were the exact same as at the temples (actually it was a few thousand cheaper - I think they buy group tickets and just resell them, but they work and the prices were not any different than the foreigner ones at the temples nor online)