Disagree here. I live in a very expensive area. Also part of a relatively conservative catholic church with lots of large families. We have three little ones and that's below average. I can usually find a 15 year old for $12 an hour. They are very experienced because they have siblings and mom and dad are usually just a few blocks away. When they go to college they usually want more money, but there's a neverending supply of 15 year olds.
For $25 an hour, I'm pretty sure my wife and I would take turns going out for the next four years until the oldest turns 10. $100 bucks for four hours, probably two of which the kids are asleep anyways? No way.
I do agree that market rates are what they are, but nothing wrong in wanting to find a good deal.
A bit of a necro post, but I'll reiterate.
You get what you pay for.
Yes, you can find a 15 year old for $12-15 an hour.
There aren't a lot of them. Many 15 year olds don't want to work, or don't have to work.
Also, many 15 year olds are not terribly experienced.
What do you want? Do you want a teenager to play with your kids for a couple of hours? The last teen I hired, we got home at 11 pm, the 18 month old was still awake, had cried all night, and was wearing a diaper backwards. The 7 yo fessed up that the kid cried all evening. Two weeks later we hired the 30-something. Got home at 10, both kids asleep in bed.
Also, as mentioned above, we were talking about an infant and 3 other kids under the age of 5. That's a LOT of kids, mostly likely too much for all but the most experienced teenagers.
For the record...we almost never went out, for years. Until we found that great babysitter.