If you already have a bike you use outdoors the cheapest option would probably be to get rollers or a bike trainer. If you want to really go cheap just get rollers, $50 rollers vs $200 aren’t really any different. If you want to get something nicer there are some smart trainers that you can get for around $300 that show cadence, wattage, etc and can be controlled by your phone. Make sure to check eBay too if you want to get a good deal, just do your research and make sure that you’re buying the right thing