I found this website on the net. Ubud is about an hour's drive from the airport. I have no personal knowledge of this company but the rate seems ok.
There is a taxi office at the airport where they have a board with the transport rates listed. Taxi fares generally compare very well with other modes of transport. Generally the blue taxi's are considered the best - privately owned I believe - they are pale blue.
Anyway this link to Ubud Taxi company quotes 125.000rp which seems reasonable, I don't think you'll do it for much less unless you have lots of time and energy to search and bargain. As I've said though, I have no personal experience with them
Catherine
http://http://ubudtaxibali.web-log.nl/