Just looking at the financials they are pretty comparable with PL relegation fodder/Championship promotion seekers.
But again, IF they were allowed in the Premier League, they would probably be in a position to invest a lot to stay there, and if they did manage to establish themselves would have a platform for growth with their passionate multinational following, large market, big stadia, etc, etc. They're giant clubs, they just can't attract any TV audience for games against anyone but each other, and TV money runs the game now.
I would be in favor of letting them in, personally. Along with any other Scottish clubs that wanted to join the English pyramid. I think the relationship Wales has to the English League is better for the game there than the way Scottish football is withering and dying.
And a Premier League with Rangers and Celtic would be a stronger league and a stronger TV property which would ultimately benefit the whole league as against La Liga, Serie A, etc.
I often wonder about the practicality of away day travel though..... Harsh on the dedicated match-going Scots to have to travel all the way down here every fortnight.
A tough pill to swallow; especially for commercial reasons.