Mandala now offers on-line booking that includes being able to pay for flights. This is only very new as they weren't doing it as recently as last week.
They fly both DPS-YOG and YOG-CGK.
You should have no trouble getting both legs for around 500,000rp per person. Or about US$27 per leg which is a very good deal.
This is the website
http://www.mandalaair.com/
Mandala have just been classified in the top band of Indonesian airlines with Garuda and Merpati. I've flown them about 6 times and havent had a problem.
Air Asia are good and cheap but they don't fly either of the routes you mention.
The only airline I would actively avoid is Adam Air. They have dreadful reputation partly because their owner is a prominant politicain who manages to "get around" a lot of airline regualations. The locals call them Adam kapal selam - Adam submarine, which doesnt really inspire confidence.