If you are currently backing 3-5 horses per race - what is your strike rate and your POT.
Breakdown what you have into what you want and then you have what you need to do.

If your SR is 100% and you make a $1 per race you need 500 races per month. IS your current betting system going to allow you 500 races per month? That's 16 races per day. If not you will have to increase your stake. Is your SR 100%? If not you will have to increase your stake.

If I am reading you correctly, you are asking for an alternative method to generate this income. If so lay. Lay the field all races. It will be a bumpy ride and you may lose your $500 in 2 days. You will lose 4 months in 12 but you will win. My estimate would be $7500 per annum at $30 liability. You could also drop some of the field, restrict the fieldsize and the minimum price range.
Why have I given this? Because you won't do it. The first bad run and you will stop. $30 min liability will do that. If you could start with $1 everybody would be doing it.