Space In Colocation Facilities

Perhaps some of you who run colocation facilities could help me with the following questions

1) How many of your customers running applications on upwards of 30 servers use regular clunky server towers as opposed to nice 1U servers?

I ask because from a cost standpoint it's important to consider deploying an application on commodity hardware as opposed to expensive blades or other tiny servers. But I'm wondering if for large and growing applications that use upwards of 30 servers in total the savings in hardware purchase would be offset by the colocation facility space required by regular server towers or other large servers?

2) Also, in a rack configuration I know how the servers would be laid out. But with regular server towers, how would these be laid out within the cage? Can they be stacked, or would that not be viable because they're a lot heavier, and each server probably needs room to breathe due to heat generation.

3) What's the industry-standard pricing model for colocation? Is it based on a number of square feet, a number of cages, a number of rack units, or something else? How does this play in to the use of regular server towers?