Prices of aquatic plants range from really cheap to expensive. A few strands of anacharis will run a dollar or so. Turtles usually love it and it's really good for them, like what they eat in the wild.
The downside is that all turtles like to uproot their plants. They'll eat part of it but if there is too much in the tank you'll wind up with little pieces all over the place.
I try to manage this by not putting very much in the tank at one time. That way Spot eats it all. I'm starting to grow my own in a separate little tank so I won't have to keep running to the store for more.
The pet stores sell water plants and you can get good prices for them on-line. Here's a link I've used and it's very good:
https://www.liveaquaria.com/product/cat ... pCatId=768