over an extended period of time the agent fees will add up. My next renewal will cost me just 1,900 baht. And, I'll probably switch to the income method.
used an agent for my first year in Phuket, I even had my own funds.
When trying to renew the second year on my own I was told, rather strongly, that I had to drive 5 hours to the province where the agent had setup my initial visa.
After a 5 hour drive I was told by that immigration office that I had to drive BACK to Phuket and do it there.