How much would it cost to have a server/gaming PC in one case?
You would need to get one of those cases which support both a mATX (or sometimes full ATX) board in addition to a mITX one hidden somewhere inside.
Got any recommendations?
I'm a fan of the Phanteks Enthoo Mini XL DS, which is decently priced for this type of case, but I believe your main board needs to be mATX (which is fine, Z370, X370, and even X299 have mATX boards. Only Threadripper doesn't)