This is going back a bit, but it eventually concerns the whole misunderstanding of Gen. Tilney thinking Catherine rich. Is this a case where John Thorpe made up an idea of them being rich, and then believing in his own lie?

Thorpe knows James well, we are to assume, from Oxford. I'm sure that the reason he brought James home to his family was so that he could meet Isabella and hoped that Isabella could marry into a rich family. But why did he assume that the Morlands are rich?

From chap. 11:
“If your brother had not got such a d — beast to drive,” said he soon afterwards, “we might have done it very well. My horse would have trotted to Clifton within the hour, if left to himself, and I have almost broke my arm with pulling him in to that cursed broken–winded jade’s pace. Morland is a fool for not keeping a horse and gig of his own.”

“No, he is not,” said Catherine warmly, “for I am sure he could not afford it.”

“And why cannot he afford it?”

“Because he has not money enough.”

“And whose fault is that?”

“Nobody’s, that I know of.” Thorpe then said something in the loud, incoherent way to which he had often recourse, about its being a d — thing to be miserly; and that if people who rolled in money could not afford things, he did not know who could, which Catherine did not even endeavour to understand. Disappointed of what was to have been the consolation for her first disappointment, she was less and less disposed either to be agreeable herself or to find her companion so; and they returned to Pulteney Street without her speaking twenty words.

This seems to indicate that he already assumed James to be "rolling in money". He couldn't have thought that they were going to be rich because of a possible legacy from Mr. Allen when he only found out about the Allens' being rich after talking to Catherine in Chapter 9. There is no reason where James would tell Thorpe that they have rich neighbors at Oxford.

So did he only think that the Morlands are rich after he found out about the Allens in Bath? This seems unlikely, since isabella already showed an "attachment" to James as soon as the latter arrived in Bath.