Hi Arthur,
As one who has spent a fair bit of time in recent months keeping an eye out for various bits for my own RB67 Pro S system, I'll chime in...
As always, selling as a complete package is most convenient for the seller if you can find the right buyer and are willing to (generally) accept getting less than you'd probably make selling separate pieces. If you're not in a hurry I think you'll see a better return if you part it out.
If you haven't already done so, I would suggest do a search on ebay for the various items to get a sense for what they usually go for. Also, check out KEH's listings as well.
My recollection of some recent auction activity for a few of your items:
RB67 Pro S, w/WLF, 120 back, and one lens (usually 127mm or 90mm, C or non-C): ~$350 (sometimes under $300).
180mm C lens: ~ $150 - 200.
Beattie screen: last week one went for about $80.
Non-metered prism: ~$75
Of course on ebay, it can be all over the map price-wise... a 250mm non-C went for $42 the other day, then another for $89, etc.
KEH's listings are also helpful in showing a range of reasonable prices for gear in various conditions (according to their ratings system, anyway).
- James