ARIN charges an annual fee for membership ($500) plus you have to purchase the IPs. The smallest block they offer is a /24 and they require justification of 100% before you may request a second block. They do allow 80% unassigned for growth.
Their justification basically means you must have a valid reason for using a single IP with the domain vs using host headers. A valid reason is something like an SSL. Using 1 IP per domain is not a valid reason.
If you can get them inexpensively or free from your colo provider you will be much better off.
Brad @ Xiolink
XIOLINK. Your data...always within reach.® http://www.xiolink.com
1-877-4-XIOLINK [+01 314 621 5500]