Surprisingly little. Here's the deal:
Single User Licenses
xv is shareware for personal use only.
You may use xv for your own amusement, and if you find it nifty, useful, generally cool, or of some value to you, your registration fee would be greatly appreciated.
Commercial, government, and institutional users must register their copies of xv.
This does not mean that you are required to register xv just because you play with it on the workstation in your office. That falls under the heading of 'personal use'. If you are a systems administrator, you can put xv up in a public directory for your users' amusement. Again, 'personal use', albeit plural.
On the other hand, if you use xv in the course of doing your work, whatever your 'work' may happen to be, you must register your copy of xv. (Note: If you are a student, and you use xv to do classwork or research, you should get your professor/teacher/advisor to purchase an appropriate number of copies.)
$35/unit perpetual for major version (3.x).
Commercial (includes bundled/embedded/integrated):
$150/unit perpetual for major version. Annual license available (for site licenses only) based on 40% of perpetual license rate.
You should purchase one license per workstation, or one per xv user, whichever is the smaller number. xv is not sold on a 'number of concurrent users' basis. Given that xv is completely unlocked, there is no way to realistically enforce any 'number of concurrent users' limits, so it isn't sold that way.
Checks, money orders, and purchase orders are accepted. Credit cards are accepted, but currently only for a single-user license (multiple copies, or site licenses). All forms of payment must be payable in US Funds. Checks must be payable through a US bank (or a US branch of a non-US bank). Purchase orders for less than $50, while still accepted, are not encouraged.
If you are planning to purchase 10 or more licenses, site licenses are available. For sites of 15 or more licenses, discounts apply and become increasingly substantial for larger sites. Site licenses let you run xv on any and all computing equipment at the site, for any purpose whatsoever. The site license covers the current version of xv, and any versions released within one year of the licensing date. You are also allowed to duplicate and distribute an unlimited number of copies of the xv manual, but only for use within the site. Covered versions of the software may be run in perpetuity.
Also, it should be noted that a 'site' can be defined as anything you'd like. It can be a physical location (a room, building, location, etc.), an organization (a workgroup, department, division, etc.) or any other logical grouping ("the seventeen technical writers scattered about our company", etc.). The site license cost will be based on your estimate of the number of xv users or workstations at your site, whichever is the smaller number.
If you are interested in obtaining a site license, please contact the author via electronic mail or FAX. Send information regarding your site (the name or definition of the 'site', a physical address, a fax number, and an estimate of the number of users or workstations), and we'll get a site license out to you for your examination.