You're right. But I think it will be 60m+ addons which will bring the price to 75-80m. I can't see us paying more than that with the current economic climate.
Tbf I think the reason we are spending so little is because of our enormous payroll. We have one of the biggest ones in the world. And with players like Henderson and Lingard on close to 100k a week, it's no wonder.
If we were to reduce our payroll by 25%(doable) we would have an extra 50-60 million every transfer window. But that would mean that Woody and Judge would have to get their act together and be competent for once. I'm not holding my breath though.