China Eastern has direct flights from Kunming every Tuesday, Thursday, Saturday and Sunday...on other days also connections from Kunming via Nanning possible. For May I found prices as low as 700 RMB, but you need to add slightly more than 500 RMB taxes, which is all in all still very cheap.
Yes, you can get a visa on arrival on the Mohan-Boten border. If possible, pay in USD (30 USD). RMB or THB are also accepted, but at very bad rates. Take a picture of you with you as well, otherwise they charge a fee, normally just 1 USD, so not a big deal. On weekends they charge another 1-2 USD for their "weekend overtime";-)
Anyhow, can you provide your nationality to make sure that I didn't tell you anything wrong...?
I'm planning to go to Xishuangbanna over the National Holidays. If anyone is interested to join the trip, please feel free to contact me.
About myself, I'm a 23year old German student at Yunnan University, just arrived in Kunming three weeks ago. Studying Southeast Asian Studies, I'm deeply interested in the Xishuangbanna region.