FWIW
I deal with customers on both sides (leasosr and producers) on a daily basis. It sounds like your mother would benefit more from an upfront, fixed amount of money (cash rent). Like others have said, do your homework. County FSA office can provide you with yield history. Ask, as many knowledgeable people in the area as possible, rent prices on cropland. It sounds a little low for the area, but again there are many factors affecting this (ie: delta or hill land, land leveled, irrigation, commodity prices, yield history). Like buying or selling any type of real estate, find comparable leases/rents before you negotiate. If your price truly is fair and the current producer does not want to pay, then I guarantee someone else will.
As several have stated, you need to remember the producer in this situation also, especially if he has been a good tenant. Just because soybean prices have gone up 50% in the last yr, does not mean your rent should.
Also make sure you know who is getting the government payments on the proeprty. Although they are probably very minimal now, you still need to check.
If you ever inherit the property and are looking for a good ROI, you may want to think about share rent. Obviously there are cons to this, such as growing conditions and weather. On t he other side of the coin, taking on some of the risk allows for better returns. Also, to offset some of that risk, you can purchase crop insurance yourself (buy up additional coverage over the producer's policy) that will protect from a total loss in adverse years.