Re: Is the 1520 a fair trade with an 830
FWIW - I don't think a cracked anything is going to be a fair trade for a mint 830.
On the 1520, 930, Icon, and on and on, it is very, very, difficult to replace a cracked screen with anything other than a completely new LCD/touchscreen assembly. For any of the above phones, that assembly is around $150 all by itself. So, you can subtract the cost of the screen from the value of any of those phones right away, before you even test their functionality, or value their other wear and tear.
I wouldn't trade a mint 830 for a cracked screen 930, and I think the 930 is about the best phone ever made. But, that's just me I suppose. The repair cost is just too high for me. Someone who intends to use it as is might pay more.
The value of a 1520 with a small upper crack is entirely in the eye of the buyer. Some people wouldn't pay $40 for it, while others would pay $150. It depends on what the buyer is going to do - refurb it, or use it as is.
Like Xandros says, if one of them is unlocked, it may change the value a bit.
All other things being equal (condition and functionality), a 1520 is worth more, and is a better phone. Cracked screens kill value, unfortunately, because they are so expensive to replace.
Just one fool's opinion.