Being that I work in an actual nerd store, I can answer some of these questions, I guess in reverse order:
Unhinged lands (you said Unglued but posted pics of Unhinged lands so I'm gonna assume those are the ones you're talking about) are worth about $3. Naturally, Island is worth slightly more due to the fact that it taps for the best color of mana in the game... that and it looks f'n sweet.
As for the prices on dual lands/power, that's trickier to answer. There's really 3 answers- there's the sale price (what places like Star City Games and the place I work for will charge you to buy it), the buy price they'll give you for one and of course the ebay/individual seller price, or what I like to think of as "the real price". To further complicate matters, the condition of these old, valuable cards matters a lot to the people who are gonna buy and sell them, so if there's any imperfections it's gonna tank the value of the cards. SCG sells a NM Beta Underground Sea for $1200, buys them for $700. Normally I'd say you could split the difference if you're lucky on ebay, but I watched a nm Beta Sea go for about $1400 the other day on ebay. The market for beta duals in particular is pretty crazy right now due to the spike in prices on legacy cards that happened in February and kind of keeps fluctuating through present times.
Gundam- if you're ever serious about selling any of your stuff, let me know and I'll talk to a guy I work with who will probably give you a very good deal on any decent looking beta duals you have.
As for my most valuable card... well, in a way it's the Nissan Sentra I'm driving.

Magic cards financed the purchase of that piece of machinery. Actual cards would probably be... sadly Koth of the Hammer I guess. Sold all my legacy crap to buy this car and move.