Referral fees as an indie dev?

Subcontracting has its own headaches and overhead. If your goal is to continue being a consultant then by all means, sub it out and run the project.

If your goal is to focus on building your own projects then you're better off finding a dev shop or agency to refer it to. It's perfectly OK to ask for a referral fee, in fact you should insist on one. In my experience the normal rate for a referral is 10-15% of the contract value.

Coming from the civil-engineering background, the concept of sub-consultant is very common. You only cherry-pick the tasks that you wanna handle and sub out the remaining tasks to the other guy. This has multiple benefits:

1. Your client trusts you, not "the other guy you trust", it's better that you still be the point of contact and be involve in some part of the project as a leader. This gives your client a sense of confidence and you will be in control of the project.
2. You will know what's going on in the project. Your reputation is on the line here even when you refer someone to fulfill the project. (Think about what happens if you refer to the wrong guy, your client won't be happy with you either).
3. Should your client need any maintenance that you can actually provide down the road, you will have the necessary knowledge to provide the maintenance, and the client would not ironically bypass you and communicate with that other guy directly.
4. You earn more money this way.
5. You gain one more project under your portfolio. Bigger project on your portfolio is always nicer.