it's not about the money. many mappers are here to build a portfolio and meaningful professional experiences. most people here don't care about the money, we just want to contribute to the game because we enjoy the tf2 community.
If you are serious about building a portfolio, why wouldn't you pick the unreal engine? It' sway more portable and integrates with tools that professionals actually use in the industry. The SDK is also kept up to date and much better than source SDK.
You could also join dev communities for Xbox and other consoles and whatnot if you are serious about promoting your work.
If you want a career, don't box your self in and settle for an old/dying engine... Tf2/source is a niche community, and won't get you far.
Idtech would be another decent option, and it's very similar to hl2 due to to the fact that HL was built around this.
The only thing that I can see that's good about TF is that i'ts F2p and has a large client base.