Not really. In terms of cost: hardware nowadays is quite cheap, US$500 can get you a decent machine without a monitor. BuildWise server is free and BuildAgent is as low as $22.5/month (with renewal discount). Unless you are building the next Facebook or Twitter, the chance of your test execution time is in a matter of hours, which is very admirable. 4 agents can reduce 2 hours' test execution time to around 35 minutes.
In terms of effort, setting up BuildWise is simple and quick, so is the BuildWise Agent. Of course, like any CI solution, it depends on the nature of your application. For example, a 2GB-memory Linux VM is enough for running web testing (in Chrome). The total cost of 5-agent build farm in Vultr or Linode (both are reputable PAAS vendor): $10 x 5 = $50 per month. If you want to keep the agents 24x7 running without the need to restart, add $22.5 (with renewal discount) x 5 = $112.5 for 5 BuildWise Agent licenses. If you want to run tests on Windows (10 or Server), which are the more common platform for our clients, there will be extra for Windows licenses and bigger VMs ($30 more for each VM on Vultr).
Finally, once your build farm is set up, you can easily configure it to support another project. The resource is fully reusable.