There are tons of those around (not all Miyatas, but hundreds of thousands of bikes from that era and about that quality). I've seen them in garage sales as low as $5 (needed work, but I got it running for a neighborhood kid for less than $35 including a new chain). $50 to $65 might be typical around here, but really, it's worth what somebody will pay. If you're the buyer, have no interest in working on it, it's ready to ride and you want to get on and pedal away, maybe $100? If you're willing to put in a few hours work and learn some new skills, you can probably find something equivalent for $25 or less.
If you're not used to working on bikes, don't let that scare you away. They're pretty simple and there's lots of information available in books and online.
If you do buy it, pay some attention to setup. The bars are slanted down too far in front, the brake levers are in an inconvenient place and the seat is tilted down. Five minutes with basic tools could make it a lot more comfortable.