Looking for a Nether Void/The Abyss and questions about trading

I'm currently looking to trade for a Nether Void and The Abyss and was wondering how trading up in value usually works.  For example, if I was trading all $5 cards would it be usual for me to trade above the value of the one valuable card I'm trading for?  I have a Tropical Island that would trade fairly evenly for one of them (a little extra for the abyss) but I'm wondering about all this for potential trades for the other.  Thanks a lot for any input!