With respect to the cost to upgrade, it's not something that one can purchase separately. DVDA 5.2 is paired with Vegas Pro 10. If you purchased or upgraded to VP 10, DVD Architect 5.2 is available at no additional cost.
I don't think that there is too much difference between features. I had problems using DVDA 5.0 with output from Vegas 10 once so I concluded that you should try to use the versions that are related for best compatibility, but your milage may vary from mine. (That is: use Vegas 9 with DVDA 5.0 or Vegas 10 with DVDA 5.2)
When 10 first came out, there was a special price for a short while, but only some people were emailed about it. There is speculation that a new version might be announced at NAB, so I would wait to see if that happens and look for a special deal soon after.