I think one have to build his or her income before thinking of discretionary income, it's after you have different streams of income that is when you will have a stable income and then discretionary income follow suit, that is when an investor can invest his or her income wisely
You must not have different streams of income before you get started in bitcoin investment, all what you need to invest in bitcoin is to be able to figure out a discretionary income to start accumulating and invest with. I’ve always advised newbies that it’s better you get started in accumulating bitcoin once you’re desirous to invest in bitcoin as far as your discretionary income to get started with is available, and by the time you get started in accumulating bitcoin you can also decide to increase your income sources if you have the opportunity to do so in order to keep you afloat with your ongoing investment with a long term plan of consistently and persistently accumulation of bitcoin and hodl and gradually build up your portfolio. That time you’re waiting to build different sources of income before getting started in your accumulation would have been used to accumulate a reasonable amount of bitcoin stash and build up your portfolio gradually, and most times, those who waits or procrastinate usually end up not doing anything and abandoning their investment ambitions.