According to Bible, is there any standard for defining the salary level of interns, preachers, or evangelists? Also, in the business world, it's common for companies to award employees one-month salary bonuses. (For some very profitable businesses, it may be more.) Is this applicable to church staff? In your opinion, is this appropriate?

As to your first question, t
here is nothing in scripture on this.  What we see is:
* 1 Corinthians 9, Matthew 10 -- those who preach the gospel should be supported.
* Level of support never specified -- bare minimum? comfortable living wage?
* In the early church, leadership appointments seem to have been for life. That had a good side: a preacher was less likely to compromise or even slant his message in order not to offend those supporting him.
* Paul and others apparently from time to time waived their right to be supported, although they did not force others to follow this example.
* There was no professional clergy class.

As to the second, I was once part of a church staff where a bonus was given (only once). It was maybe a quarter of a month's salary, and wholly unexpected. The other 19 years I was on staff there was nothing.  This is not really a biblical issue. I think this is fine, or neutral. Many cities are expensive places to live in, and church workers' lives are far from normal. There are lots of additional expenses, often unreimbursed.
Of course if many or most of the members supporting the staff are hard-pressed, this may be inappropriate. As Paul wrote, "Our desire is not that others might be relieved while you are hard pressed, but that there might be equality." This is not a socialistic principle. It is about fairness, sensitivity, and (ultimately) love.