DC prices isn't something that's posted publicly.
As for what can and can't be requested for items can be found
here.
As for what you can and can't get on top of other stuff on DC items, depends entirely on the item itself and what you're requesting to have added. You can however search the archive for similar DC requests to get a feel for what has been approved or rejected in the past, though the teams position on various properties or DC item power levels have changed over the years, just as most of the server has in terms of server balance. As such, some items that have been approved would never get approved today, while others are on the low scale of what you might be able to get.
If you're not sure on what powers you can get or not, then you can always "overbid" on the properties you want and if it's too powerful, you'll get told so and likely get some suggested changes that would be fine. The downside of doing this is that your request is likely going to take longer than normal to get approved, since it'll require a discussion.