Most proto auctions have been running full term without issue lately, I don't think he'd have much of a problem if he went the EBay route. There are several on there now that have been on for some time.
As far as price, I'd figure between $75-$200 for everything aside from the Solomon's key 2 NTSC proto which should net you significantly more depending on who was interested.
I recently purchased a less than desirable proto in Tiger Heli for less than $80 shipped in a well advertised auction:
http://cgi.ebay.com/ws/eBayISAPI.dll...=STRK:MEWNX:IT
And I also recently won a higher profile proto in the form of the this Ducktales 2 for $275 shipped in a well publicized auction that ran full term:
http://cgi.ebay.com/ws/eBayISAPI.dll...=STRK:MEWNX:IT
And this Super Spike V'Ball proto has had several price reductions and is still sitting on EBay for $159 BO just to give you an idea of what to expect on low profile protos:
http://cgi.ebay.com/ws/eBayISAPI.dll...=STRK:MEWAX:IT
Those are just to give a context to my estimates as my numbers are based around sales of NES protos for the last 2 years. Sometimes you'll see released games go for higher, but unreleased game prices (as in unreleased in a certain format but released elsewhere) can be dependent on who is looking and when. I just recently purchased a Super Turrican NTSC proto for $325 shipped, but that is the only recent sale that I can think of that is similar to your Solomon's Key 2 proto.
You sure the Caveman Ninja cart is NTSC? That's the PAL name and it says "Euro" on the cart.
Very nice finds, cool to see so many protos from so many different companies end up in one place at one time. Likely from a reviewer or magazine maybe?