If you know any independent CFIs in your area, you might ask them what they charge. Most flight schools will pay their CFI a bit less than what an independent CFI makes.
The going rate I've seen is usually $20-30/hour, depending on region and pay scheme.
Retainer pay - I'm assuming you mean some sort of guaranteed minimum weekly pay agreement? These agreements are rare in the CFI trade, unless you work for a large, highly profitable school. Mostly you get paid when the prop is turning.
I am technically a part-time employee of my employer, so I don't get any benefits but can take vacations as much as I want so long as I find CFIs to cover my students while I'm gone. Don't worry - you won't have the money to take many vacations anyway.
