How much it cost shouldn't have any bearing on whether Ron wanted a new television or not, but rather whether he was going to get one. Even if she couldn't get a straight answer out of him, Hermione deduced that yes he did want a bigger television, but couldn't work out how much he would need to spend on one. Unfortunately she couldn't work that out either, unless she could see what he was talking about.
"We are saving for a house and all," she assured him. "But if you want to go and have a look at televisions then I suppose we can." She wasn't sure she saw the point in taking out any amount of money now that he wasn't certain he was going to use, however.