Under a hundred dollars a night for sure - just 3 of us, but our son is little and doesn't take up much space yet.
I'm not that familiar with Ubud so I can't recommend a specific place but you could rent a reasonable quality two storey villa in Seminyak with full kitchen and internet not far from the beach for @ $US30/night. For US$100/night you should have no trouble finding something..
The internet might be the sticking point. Fast ADSL and 3.5g internet access are only just becoming available in Ubud so it's unlikely that you will find a rental place with it already installed. You might find somewhere with dial-up access or a shared radio connection but other than that you are probably going to have to use an internet cafe.
Let us know how you go.