I would check out a local builder.
Figure $60 per hour for 2-3 hours (depending upon complexity).
One way to judge the local builders is by the duration their business has survived. The NEW guy in my town has been in business in a storefront location for nearly 10 years.
Other wise, check systemax.com Systemax is the parent company of TigerDirect. The systemax computers don't have a lot of frills or options, but it is a good way to judge the competition on price.