10-20 for domestic coach, 20-40 for domestic first. For instance, most medium haul flights are about 11 bucks coach and 19 bucks first. So the mark up for first is about 8 bucks. If its for leisure and you get one drink and/or something to eat it has already paid for itself AND you get slightly more comfortable seat. Domestic first is not as likely as the upgrade list is usually quite a few people long but it does happen - between me, my spouse and my parents its about a 50/50 chance of first vs coach.

International is where it adds up but the cost can be very worth it - getting business or even first class is much more likely and while it is a bit more in price - it is SO worth it for making the 8-16 hour flight that much more enjoyable.