So, I ran the Philly half last year & just signed up to run it again this year -- being one of the first 1K to sign up, the cost ended up being $77 this year ($70 registration, plus fees - price goes up to $120 by fall, I believe). And you know what? That's clearly an expense that's a luxury and not necessary, and I would never suggest it's prudent for someone with debt to be signing up for these expensive races left & right.
That said (and I don't think anyone who hasn't done such a race will really get it, I totally didn't) - doing such a large race with so many people is *SUCH* an amazing experience. Yeah, I can run 13 miles on my own today (and most of my runs are solo & free obviously), but it's completely not the same. You're in this huge mass of people, running & getting to know a city in a different way, with tons of people cheering you on everywhere on the sidelines. WHAT an adrenaline rush and community feeling. So when I think about everything that's entailed in this race -- closing part of the city down for an entire morning, volunteers who take your clothes/items at the start, water and gatorade stations, the nifty shirt you receive, all the organization/security - I have no qualms spending this money.
Though I agree that for shorter races (5Ks), or if you want to run races a lot, it could be mustachian to pay for official races only a couple of times a year, and to try to organize something informal with your friends/community instead for the rest. (Also, 5Ks are so much easier to set up -- need not much space, many more friends & family would be interested & able to do that). I used to live in DC and I know that Arlington, VA used to have free runs on Friday afternoon/evenings during the summer, I wouldn't be surprised if lots of other cities had something like that.