Can I extend my 60-day single entry visa for an additional 30 days at any immigration office in Thailand?

1900 baht at the immigration office in your area

you didn't forget anything, :O You can't "upgrade" at the airport. :/

You get a 30 day EXTENSION at the immigration office in the area where you are staying for 1900baht.

In Bangkok, Chiang Mai, Phuket & Buriram you can apply for a 30 day extension when you have 45 days or less left on your current stamp (so 2 weeks after you get here). At other immigration offices you can do it when you ahve 30 days left on your current stamp

Yes, You can extend it for 30 days..