Solving this problem with either Lotus 1-2-3 or Solver takes about the same amount of effort. (It just takes more work to describe the process in 1-2-3 than it does in Solver.)
So what's the advantage to using one or the other?
Certainly Lotus 1-2-3 produces graphs that are more presentable. With a little effort they can be printed and framed. However, for my money, the power of the HP CALC graph outweighs the prettiness of the 1-2-3 graph -- The advantage is Solver!
Moreover, if I want to see how much profit I'll make after 100 people have registered their copy of "The Diversions Disk", all I have to do, in Solver, is store 100 in Units and solve for GrPrft.
If I try the same thing in 1-2-3, it won't work. Trying to put 100 in cell D10 either gets me a warning that the cell is protected or the 100 overwrites the formula in the cell thereby destroying the worksheet. Again, advantage Solver.
Solver gives completely interchangeable solutions. With Solver it's possible to solve for any variable in terms of all the others.
Lotus, all by itself, can't do this.