If your relatives or your friends want to extend their 30 days tourist visa in the Abu Dhabi Emirate they have 2 options. You can go to the Residency & Expatriates Affairs office and pay 700 AED or You can do an Oman Visa Run and pay 35 AED upon UAE exit plus 5 Omani Riyals (48 AED) upon Oman entry. Oman exit and UAE entry is free of charge.
I chose to do the Oman visa run from Al Ain (eastearn part of Abu Dhabi Emirate) through Mezyad Border for my mother-in-law. Not only is it cheaper, but you can repeat this process without restrictions.
I have started the trip at 5:30 AM and have reached the Mezyad border at almost 9 AM.