That’s not a question you should ask; however, it is the first answer you need.
If you ask a senior engineer what’s their rate, he would probably be in the range of 80-100dls/hour. In your current company stage, can you afford it?
Leaving this question until the end might be wasting your and the engineer’s time. If you can’t afford the engineer, there is not much to discuss.
A far better approach is to be upfront about what you can afford. “I’m a first-year bootstrapped company and can pay $50/hr. Does it work for you?“
Then you will attract the exact type of talent you need now: a senior engineer who can see growth potential in your company and is willing to help you.