This rate can vary greatly depending on various factors. Some factors are the region you are located where rates are high in NY, LA as compared to Taylor, Nebraska. The expertise of the individual does affect your rate as well. Are you planning on upgrading the existing hardware from 2000 to 2003 or is there another server that will have 2003 installed and then integrated into your infrastructure. Are your clients at minimum 2000 Professional or do you have some legacy 95/98 systems. That’s a factor. For your size of environment, I would give a flat fee so it protects you from “overage” of hours due to issues occurring with the upgrade process but it protects the IT individual as well.
You sound like a small shop with 1-2 Servers with your 20 users. I’ve seen an upgrade for this type of environment performed as quickly as 2 days (with no issues) and as long as 5 days (whole business week with issues).