Originally Posted by halfspeed
There are two different 720s. After the classic touring bike went out of production, a lower end hybrid was introduced with the same model number. Don't get the two confused.
The original 720 has become a bit of a collector's item and is often priced to reflect that. While the 520 doesn't carry the collector's premium, it still is usually overpriced because of the reputation the modern version has earned. The 80's 520 was more an entry-level bike and while there's nothing wrong with it, it's not something I'd pay more than $25-$50 for depending upon condition.
I ended up with a 620 which is a much better bike than a contemporary 520 without the collector's premium of the 720. That it's also the least known model also helps keep its price within reason.
I'm not sure I would call the 520 it an entry-level bike (Mid-level?). Mine was equipped with Shimano N-600 components throughout. The frame is Reynolds 501 instead of 531 but that has never been a concern of mine since I'm a bit of a clydesdale.
I want to find an older steel Trek with long chainstays. My first choice would be a 720 or a 620 like yours halfspeed. The other bike I'm thinking about is the 1987 Trek 520 with 531 reynolds and 45.5 chainstay length.
I see by your profile that you have two nice vintage Treks.
The one thing I notice is you have to pay attention to braze on attachments for the older Treks. They seem to be different for each bike.