Most fire departments will also give them to you for free. My brother is a professional fire fighter and always has a box of them in the back of his truck to hand out to people. They'll even do volunteer events where they'll come and inspect your house to make sure you have proper placement and they're not too old.
At $60 each I might replace the 5 I have in my house, at $130 I don't think so.
A photoelectric smoke detector seems to be around $20 on Amazon and CO alarm is ~$30, so you're right - it's between 2-3 times more. I was basing my comment on the actual pricing seen on the shelf at hardware stores here (DC), which is higher.
That said, it's hard to compare the base price without starting to talk about features: i.e. the cheaper detectors on sale around here tend not to have things like the LED needed to tell which of 5 alarms is currently going off. That part costs a few pennies but the pricing jump to get it much higher, at least for what was on offer at my local Home Depot last year when we replaced everything.
As other people have pointed out, the Nest detector uses photoelectric detection and multiple of them will link together. Comparing it to a $20 ionic detector is apples to oranges.
