To extend your visa from 1 month to 3 months will cost you somewhere between $20 - 100USD depending on your country of origin. If you want to stay longer you will probably have to ordain. To extend your visa from 1 to 3 months requires a lot of waiting around in a crowded, hot and humid room with a variety of pushy Indian people and disgruntled western tourists for a good 4 to 6 hours in total, there's a lot of backing and forwarding and the whole process is rather convoluted. Sri Lanka, like most asian countries can be pretty corrupt, so it really depends on who you know. If you know the right people you could get a longer visa without ordaining, but I wouldn't bet on it.
If you finish your 3 month period and you want to stay without ordaining you could also look at taking the ferry over to India and then take it back again. That might even work out cheaper than the associated costs with trying to extend your visa beyond 3 months.
"But, Udāyi, let be the past, let be the future, I shall set you forth the Teaching: When there is this this is, with arising of this this arises; when there is not this this is not, with cessation of this this ceases." - Majjhima ii,32
Nanavira Thera's teachings - An existential approach to the Dhamma:http://bit.ly/LDsGHg