We are editing a movie in Vegas and he doesn't have the dvd architect version. I'm going to get mainconcept encoder to encode the video to mpeg2. But I need an affordable DVD software option that will allow us to get the dolby digital format audio (ac3?) from the .avi and use it with the mpeg2 video. I'd love dvd architect but I don't see it sold separately.

To get DVD Architect, you upgrade from Vegas to Vegas+DVD.

https://www.sonymediasoftware.com/shopping/item.asp?PID=336&cid=111&disp=1

I'm using all my money all mainconcept so i won't have much to use to buy a DVD software and 300 dollars to upgrade is waaaay over my limit. Is there a cheaper option?

http://tmpgenc.pegasys-inc.com/en/shopping/tda20.html