The company-to-company transactions usually involve leased acreage including current production as noted by Mr Goodwin. Also, a substantial part of the value comes from not having to spend months or even years of labor-intensive effort with all of the man-hours worked by the land department to assemble acreage blocks.
Echo is having success with both minerals and leases in this area.
If you are not needing all the money but could use some money, I would tell all the companies that you are not selling your mineral rights but you are willing to sell 100% of the ROYALTIES in the Ladd 1-8-5xh WELLBORE ONLY. Operators tend to get better and better at maximizing well performance over the years. Once they do, they could come back to your section and drill 7 wells during the same year...and that is when you want to own your stuff. However, if the well is disappointing, that well will hold the acreage and it may be 3, 5, or even 20 years before they drill another infill well. All that being said, I would never tell anyone not to accept $13,500/acre... you could possibly be missing out on a ton of money/or possibly making a ton of money with any decision you make.