+1 on Mr_Wilson's Bushy kit from the other thread. It's an excellent value for the money.
A spare rifle is the best "spare parts kit", but if they both break you'll still need some spares
It doesn't take long (or much money) to have a comprehensive spares kit that can handle anything.
Here's my small parts kit. It contains enough parts to completely rebuild a couple of lowers, as well as deal with other problems:
Larger parts, like buffers, buffer springs and bolt/carrier groups are kept in a different box. This box was less than $4.00 at Wal-Mart, and it works perfectly for me.
I only have a couple hundred bucks invested in these parts. Many of them came from extra lower parts kits, and parts that I've replaced (like triggers) with updated components.
There are a lot of guys here like Stickman for example who use their ARs for work. Consequently, their spare parts kits are far more comprehensive than mine.