It depends on what the VA is doing for you. If the VA is just copy/pasting, let's say some files from one folder to another that can be automated by almost any script/bot, then $300 is too much because you can find a developer that can automate that process for a much lower investment. On the other hand, if the VA is doing some complex job, and in the process, and it's making you a lot of money, then paying the VA even $1500/mo is little.
To summarize, it depends on what kind of job the VA is doing for you and how much is helping you to profit from hiring him/her. Did the VA had previous experience or you will need to teach him/her.
Let me throw some numbers here. Some fair payment will be from 70-90% of the money that you are earning additionally from VA's help. Of course, this means that the VA is not requesting additional help from you, meaning he is knowing what he/she is doing. Another thing that you will need to consider is how much you have invested in your project, if you have invested a lot then the the share (%) can be lower for VA.