how much does it cost to rent a snow cone machine or a popcorn machine? ?

Depends on the size of the party you are having. If you are having a small party (6-10 kids) then you could pick up a home snow cone machine and popcorn machine for about $30 each. There are also home cotton candy machines you can get for about the same price.

Rentals are a little harder to come by, depending on the area you live in. Check with party planning services or restaurant supply companies in your area as costs vary by location. One firm I found in Chicago listed a price of $75 per day per piece of equipment, another in Los Angeles listed $150 per.

Failing that, do a search for local carnival concessions companies. Many of these enterprises are going into "winter quarters" and will be dormant for months. Many of these companies will come set up for a low minimum ($150 for all the snacks you can eat) or even for no up front cost and you set up a snacks tab based on the number of guests you will have.

You could get a better deal hiring one of the popcorn wagons and a staff member to run it, which will allow you to offer several carnival style treats and the convenience of having someone else handle the serving and cleanup.