This forum was pretty useful for me as I've ventured into this myself over the past 6-12 months.
From my experience, the total cost can vary wildly depending on your own business model/plan.
You can really start for next to nothing at first. I mean, it could be done for <$5k, and you can avoid upfront aggregation fees by going on a commission split model at first. This model may suit some. Probably for those that have a large upfront network and/or done something similar already.
More realistically, to lay the foundations for medium/long term success, it will take considerably more of a financial + time outlay.
A broker is quite a specialised service. The good ones really know the ins and out of lender policy and the different niches with each.
Professional bodies/Education will set minimum benchmarks for a knowledge base, but after having done it all recently, I don't think that quite cuts it at all.
It took me considerably more both financially and a whole lot of time to learn the gig before actually serving my first handful of clients.