Hi @Jacqueline Williams (I am going to work under the assumption that you're not referring to MLS properties with stated commission. Assuming that's the case your mark-up) will likely be market and ARV dependent, but this may help:
If cash buyers in your market follow the 70% ARV guideline (70% Article) then you can mark it up to the 70% ARV dollar amount. So for example, if you're buying a $100K ARV property that needs $20K of work, then the 70% ARV guideline says that the property is worth $50K to a cash buyer, which should mean that your spread will be the difference of $50K and what you go under contract for.
(You will need to know what the ARV and rehab costs though. If you are having trouble with that, it might be worthwhile shadowing an experienced wholesaler/rehabber and not worry about your "commission" today, rather focus on learning from these investors so in a short time you will be experienced and can do it on your own.)