Of course totally dependent on their level of pre-existing knowledge, level of risk with the capital they intend to spend on a startup wisp, financial partners, and what scale/scope of service area they want to do it in. Varies widely with location. And what competition (if any, other than hughesnet/viasat) exists.
I've seen people do some pretty cool non-profit community based WISP things for under 50 houses, if built right it can be quite trouble free.
The tools and equipment to do really basic 100% fiber based GPON network (or active ethernet) are also a lot less costly than they used to be... The things some guys are doing on really shoestring budgets with FTTH aerial drop cables across roofs and low cost GPON things in cities like Dhaka, Bangladesh show that it doesn't have to be super expensive. In a USA/Canada context, right of way and poles and routes are an issue (even after you're got your ROW, go price how much bucket truck/aerial fiber contractors cost, if you don't do it all in house)...
I read somewhere that 40% of restaurants started by people with no previous industry experience go belly up within 5 years? Something like that. I imagine one could easily spend $400,000 setting up a restaurant and working 14 hour days and ultimately not be successful, so doing something like a startup WISP on one's own similar amount of funds (and loans, private party investors) I would think of something similar to that. If one has the funds and the appetite for the risk and is really determined to do it, then go for it!