Those prices are outrageous. No offense, but I've seen better by a person who is actually a member on this site, and has committed more time, effort, and quality into the construction of his models.
The two on top are pretty basic really. Just basic building bricks mixed with flat pieces and a few more rare pieces here and there. A few of the others are a bit more intricate with more rare pieces. I like the designs, but yeah, the prices are way too much. I could even use the above images to build my own copy of them which would be a sinch actually.
Kevin:
I agree with you that the first one is the best of the three. The builder does have a flair for coming up with nice modern or post-modern designs in microscale.
As you stated, why pay all that money when you can build a similar structure by looking at the picture to figure out which building elements were used and then go to bricklink.com and buy the parts yourself.
Color is a very significant determinant in price for Lego bricks on the secondary market and most colors used by this builder are quit common (white, black, red, yellow, etc). There are at least 30 different colors of Lego elements. Why pay $350.00 for a red building when most of the parts can be had for one or two cents each? On the other hand, try building a skyscraper in the relatively rare color dark orange!