I always looked at the cost of tires as something you don't want to skimp on. $450-$600 is about standard on a set of quality tires installed. You may be able to find coupons or rebates as well. With tires being the only thing in contact with the road and your vehicle, best to just pay for a decent set.
I look for speed rating, wear rating, and reviews. I have had motorcycles, cars, trucks, and ev's. The one thing I will always spend a little more on are tires and brakes. Not for my own safety but for my wife and kids.
The other issue with downgrading is having to replace the tire air pressure sensors that would get you back up to the cost of just replacing the 17" tires.