Is that price just for new vinyl, or including the replacement of foam and webbing underneath?
A lot of people do go for newer seats, and if needed they modify the mounts to bolt in. Better comfort is a clear benefit, as may be superior headrests. Then again, you might need them reupholstered to match the original interior!
do you have buckets or a bench? if there buckets and there total crap not worth keeping make sure you keeps the tracks off the bottom. you can pick up nice buckets from the junkyard for a fraction of that price. mount them to the tracks and bolt them in. another option is to order replacment mustang seat covers and wrap them yourself. autokrafters sells the bucket seat foam too if you need it. but i still think picking up something from the junkyard would be your best and cheapest bet.