The answer would vary widely depending on the quality, but they were pretty pricey, especially if you were a kid. Above is a coupon from Pay ‘n Save from 1984 that will save you a dollar on Ampex tapes. Which brings the price down to $5.99.
Ampex, as a company, has a pretty storied history riddled with technology leading moments, but I never had much luck with their video tapes. Still for $5.99, if I had the money, I would have picked one up.