Sowards, 370 F.2d 87, 92 (10th Cir.1966). “Qualified and knowledgeable witnesses may give them opinion or estimate of the value of the property taken, but to have probative value, that opinion or estimate must be founded upon substantial data, not mere conjecture, speculation or unwarranted assumption. It must have a rational foundation.” Id.; see also Snowbank Enterprises, Inc. v. United States, 6 Cl.Ct. 476, 486 (1984). Thus, if a landowner’s testimony reveals that his opinion of value rests upon a legally invalid foundation, such as the price upon which he personally would insist to sell the land, his opinion would lack any probative value. Smith, 355 F.2d at 810; see also United States v. Trout, 386 F.2d 216, 223 n. 10 (6th Cir.1967); King v. Ames, 179 F.3d 370, 376 (5th Cir.1999) (<HOLDING>); Dietz, 643 F.2d at 1094 (“[W]here the owner
holding that testimony by the owners expert to the value of trees on appropriated property was properly admitted to rebut or impeach testimony by the appropriating agencys expert that the trees had no value in terms of their effect on the propertys market value recognizing that an owners opinion testimony on the value of his property cannot be based on naked conjecture or solely speculative factors holding that there was no evidence of market value where owners testimony affirmatively showed that it was based on personal value holding that reasonable inferences cannot be drawn from conjecture or speculative evidence holding that district court properly admitted property owners opinion testimony that was based on more than naked conjecture