From Black Mesa: Wiki

The Black Mesa dev team can't and will not accept donations. The main reason for this is the fact that they are using someone else's engine and intellectual property (Half-Life). The Source engine belongs to Valve and hence the dev team aren't allowed to make money from it without a commercial license. The ads on the website generate enough money to keep the site up and running. If you have the money and really want to do something with it, go and give it to some charity such as Child's Play.