What does it cost so much to buy and sell stocks?
 

Looking at the Etrade fee system, and it's nuts! You have to pay them to do anything, including a $40 quarterly "account fee" unless you had activity on your account, or had more than $20,000 in their bank account, etc etc.

Also the fees for a normal joe are $12.99 per trade. So I'm assuming that's $25 to buy and sell a stock.

Many other places have a $25-30 "minimum fee" too for each transaction.

I understand it's not that much for any serious investor, but wouldn't a whole lot more people be putting money into stocks if they could buy a few hundred bucks here and there on a bunch of different companies?

As it is, you'd really need to be buying at least $2-5k into each stock to really have it make sense.

Why such a high barrier?

Have a look at Zecco.

Those are Canadian fees, in the US, it's about 1/2 that with trades being about $6-8.
And you answered your own question, the reason is to prevent tiny trades and make sure people do some volume worth the time/effort.
WG

scottrade is 7.99 a trade