Greetings and Welcome to the Forum. We'll try to help in any way we can.
Why don't you start with some reading. Here's a good place to start. After you've read through the Basic Care info, then look at the Nuitrition, Habitat, Water Quality and other sections. After that, you'll probably have lots more questions so ask away.
http://www.redearslider.com/index_basic_care.html
To answer your specific question, the rule of thumb is 10 gallons of tank for 1" of turtle, so yes you're looking at an 80 gallon tank at their present size. However, they should be growing more in the future so they will need even more space.
On the cost, it can range from a few hundred to over $1000 for a good setup. In the Basic Care section is some more specific info on the cost of a good setup.
One thing you should be aware of - your turltes are really pretty but it looks like they've got shell problems, probably fungus. After you get oriented to the basic setup we can talk about it some more.
Good luck! You've brought your turtles this far - it's good of you now to get them the habitat they need to be happy and healthy.