I did this trip recently. As less information is available thought of sharing my experience.
The return trip to either Abu Dhabi or Dubai is SAR270. In addition you will have to get the VISA for GCC residents online. For the moment even on arrival visa is issued at the border, but sometimes SAPTCO makes an issue if you have not taken UAE visa before so better to apply for online Visa.
SAPTCO bus station is few kilometres from Batha. At the bus station you have to get a sticker for your bag at counter 5. I dont know any reason why you take this. But if you loose your bag you can may be prove.
After placing the sticker you take your bag and you have to put it to the luggage carrier by yourself, porters are available at a fee. The bus normally departs from gate 7. It is the same bus you take to Abu Dhabi and Dubai. When I told ticket checker at the reception that I am going to Abu Dhabi he didn't know that there is a bus to Abu Dhabi. So basically just tell you are taking the bus to Dubai, if someone asks near the reception. However, tell the conductor that you are getting down in Abu Dhabi, as sometimes you might be the only person travelling to Abu Dhabi.
I first thought there was just one bus. Actually based on the travellers seems like more than one bus travels on a day, all around the same time.
Female and family sits infront while single men sit in the back of the bus.
At the Saudi border the bus conductor will collect your Iqama. Then at UAE border you will have to get down and go and do the eye scan and stamping of entry.
The border and the break we took just before the border took 3 hours during Eid holidays. Altogether took 15 hours to get to Abu Dhabi.
In Abu Dhabi the bus stops way before the city. If you ask a shop owner will show you the place where you can take the bus to the city. The bus is not frequent and only travels once an hour. You can't hardly find a taxi from this place as well. But when you stand with your bags some passing by vehicle will offer a ride. Bargain and go for around 40-50 dirhams.
The bus stop in Dubai in very close to the Deira metro. Infront of Hyatt you find very small shops and travel agencies. This is where the bus stops. Next to Hyatt there is a big UAE bus station stop. Don't get misled that the SAPTCO bus stops here. As mentioned it is infront of Hyatt - near small travel agency.
Coming back from Dubai (I came back from Dubai) only took 12 hours. If it was Abu Dhabi would have taken abt 10.5 hours I guess. At UAE border the conductor collects the exit fee. You will get down and stamp your exit. At Saudi border your Iqama will be collected. Also bags in luggage and inside bus will be checked for illegal stuff at Saudi border.
When you come to the SAPTCO bus stand you can take a taxi inside the bus stand for a higher costs, or step out of SAPTCO gate and get a taxi for a lesser costs.
Hope this will be helpful for more travellers to use the SAPTCO bus when travelling to UAE.