According to the lead developer "Cman2k" on the game's "We never developed Black Mesa with money in mind. Our team is made up of average, hardworking people, and no one joined the team to make money. For us, Black Mesa is purely a labor of love. We believe this philosophy has significantly contributed to the overall quality and feel of the game. ",
The free mod which modernizes the original Half-Life with the Source engine will have a pricetag because the team will get full access to the Source engine, and also need the income.
That's not to say the devs are giving up on the free version. "In fact shortly after the Steam release there will be a completely new free version of the game. We also plan to open source our maps and some game assets to the modding community," promises the developer.