Running shoes are always updating themselves. As soon as any new technology comes forward in shoe design, a company will make running shoes that contain it. If you buy shoes with the latest technology you will be paying a premium. If you can afford it then go buy them if you wish. They would cost around $160.
You don't need to pay for premium shoes whether you're running on a treadmill or not. You pay a lot less for running shoes that are the model before the premium ones. These will have some of the latest technology in them. They will be perfectly designed for your feet. Expert runners were probably wearing them 6 months ago but now they've been superseded. They are good enough for 95% of runners out there and will cost around $100+.
The length of time new running shoes will last doesn't make a difference if they're premium or the model below. If you were doing the majority of your runs on a treadmill then they will last longer than running outside. We would recommend you replace them after 400 miles. If you do most of your running outside then they should be replaced every 300 miles.
If you take your running seriously and are willing to pay for 2 pairs of running shoes. Then it may be a good idea to have shoes for the treadmill and shoes for outside. The act of running outside wear's the shoe in a different way to treadmills. There needs to be more stability in your stride to deal with different terrains and surface. So 2 pairs does make some sense.
There are 2 types of runners who would be buying running shoes for treadmill work. There are those who are beginners who have only just started running and maybe choosing to start on a treadmill. Then there will be those experienced runners who want treadmill running shoes to do their speed work, tempo and hill work.
The experienced runners will already know what type of shoe works best for them. They are also the type who would buy 2 pairs. My advice is to stick with the shoe you would buy for outside running and enjoy doing your specialized treadmill work with these running shoes. This is the recognized advice for runners of this type.
If you are new to running and are using the treadmill as a base to enhance your fitness. Then enjoy your training with the running shoes you've bought. As long as they are proper running shoes it gives you something less to worry about. Thenb you can concentrate on getting the most enjoyment out of your treadmill runs (or walks).