Having the monthly fee is great - but something I found to be unrealistic in the beginning. For me, it started out with hourly consultation pricing. Then once you have some results you can calculate, you can start charging daily or monthly fees. Which ultimately is what you want to disconnect your time spent from what you are able to earn.
An alternative would be to make a percentage deal. But that's something I would stay away from. Especially because very often clients don't implement the advice/optimizations I am giving them. Which in turn then leads to them not having the results I was expecting.
And as always: people value what they pay for. If you consult for free, they will not do anything with that knowledge. If you get paid, they, in turn, do execute.