Hi Pat -
I am not an Attorney, but from my experience the per acre Bonus Payment negotiated by the holder of the Executive Rights would be what you are owed. The actual amount paid, of course, adjusted to reflect your Net Mineral Acreage.
If they attempt to pay you anything less than the same per acre Bonus amount that they paid the holder of the Executive Rights, then there is a strong possibility that their lease would not be valid.
If you are certain of your facts and, in particular, if you have their cover letter or other evidence of what you claim they are saying, then I suggest you consult an experienced Oil and Gas Attorney and send them that evidence.
Most Attorneys will allow you from a half hour to 45 minutes of their time for free as far as consultation time. And a nice little "Love Note" to the company regarding the issue, which 99.999% of the time straightens these kinds of situations out, shouldn't cost you much.
The peace of mind, knowing that you didn't let yourself get run over by a bunch of clods, will be worth the expense.
Hope this helps -
Charles Emery Tooke III
Certified Professional Landman
Fort Worth, Texas