You're doing a low/no/mini/micro shoot. You want to pay as little as you can, and preferably nothing. The real issue is how much your project means to you. You get what you pay for; if you pay nothing, you may get nothing. And this applies to every person working on your shoot, not just your actors.
The issue you face is cramming as much quality into your minuscule budget as you can. The solution to this is exquisite, patient planning; in other words, preproduction. Once you have your shooting script & storyboards, and have figured out all of the logistical and artistic needs you will know how much of your budget is left for paying people (or not).
You owe everyone working with you efficient use of their time and talents, and a detailed preproduction is the first step; you are showing those involved you do value their time. The next step is to keep everyone in the loop at all times, and to treat them like the professionals the are or aspire to be. There should always be a comfortable "rest" area for the cast and crew, a well stocked craft table and great meals. This lets everyone know you value their well-being - and they will work harder and happy because of it.
What you have left of your production budget after the logistical costs of the shoot and treating your cast and crew like royalty is what you can divvy up amongst your cast and crew. Everyone should, at the least, get twice the gas it took to get them there. Then you can figure it out on your own - DP, PSM and leading roles are, of course, on top of the list.