Well, we know it's going to be an April-October season, so you've got a maximum of about 30 weeks. I'd assume that April means "late April" since we aren't sadistic, so bring that down to 26 weeks or 13 home games. Add in the odd midweek or V Cup match and assume you're somewhere around 16 home matches.
For the sake of argument, let's assume 4000 attendance per match (likely high, especially at the start, but let's go with the low end of the aspiration). For reference, OFFC's first half-season at Lansdowne was 4400 *after* discounting their 16000 attendance for their home opener. Similarly, let's assume $20/ticket on average - there will probably be many tickets more than this, but likely balanced out by youth tickets and discounted season seats. That works out to $80k per match on attendance, or $1.3m per season.
Concessions and merchandising would also need to be considered, and it could be a big chunk of income. If 1-in-2 buy an overpriced hot dog at $5 profit and 1-in-4 buy an overpriced beer at $5 profit, and 1-in-20 buy overpriced merch at $25 profit, that's an extra $20k a match, or an extra $320k a year.
Sponsorship and advertising would also be another important source of revenue. According to this article, USL kit sponsors typically pay out something in "the low six figures annually". Let's spitball and say it'll be roughly equal and that "low six figures" is somewhere around $200k. Advertising would also contribute to this total, and while I'm starting to make assumptions that are way out of my experience level; 50 field-side ads at $2k a pop for the entire season seems somewhat reasonable, and $100k is a nice, round number to pop in here (if anyone has more experience involving advertising, feel free to fudge my numbers).
Stadium naming rights could also be a source of income if these teams are building the stadiums themselves (does anyone remember what the Halifax deal said about this?). Naming rights for TD Place in Ottawa is suspected to have gone for about $1.2m/year, so accounting for no football games (50%) and the soccer-football attendance difference (25%), this could work out to about $150k/year.
There's also this question about our vague friend C-SUM, whatever that is - so vague that it's not even worth putting a figure on, but could end up being an additional revenue source.
All of these together work out to about $2m in income per year; slightly more if you include naming rights in that total. Not close to being profitable, of course, but considering that part of this $5m/year budget includes "investment in fixed assets" as per the Forbes article (ie. single-time non-operating costs), it's not a complete dumpster fire either.