I am sure many people could afford $30, that is not a lot of money even in the poorest nations to be fair, not small change neither, depending on where you live that could be as high as a weekly grocery store, or an electricity bill paid, but it is still not "a lot" in the sense that it is not rent money in almost any nation, maybe 1-3% of the world would have that as rent.
However, it is still not a smart decision to put that into world cup if you don't have it. I will certainly try it, England and France and Brazil, these three look like they could actually do something and it really feels like we could end up with a good return if we wager on these three nations.
I would say $30 is not a lot, but if you do that per team then it is going to get ugly very quickly. You and I could end up gambling 10-20-30 even maybe 100 bucks on every single game, and it would still be a lot but I assume we wouldn't lose them all, so it would be like maybe 1-2k loss at very worst case, and could maybe afford it.
However, $30 for the whole tournament is not going to cut it for anyone, people who are poor enough that they wouldn't feel bad for losing 30 but can't lose more, will not earn a lot from that neither. Normally people do like 100+ odds so that they could put in 5-10 bucks and earn a lot, but world cup won't have that possibility.
As said there can be a restriction on number of bet placed but $30 is also not a small amount if you are considerably in that small gambler bracket but if you spread your bet over say 4 teams then you already have to bet $120 which is certainly big for regular gamblers as well but you will also have to see if you can manage to get back free bets so you would eventually reduce your wagers to limited number.