The Thorpes appear to delude themselves about James' material expectations.
I surmise they are counting on James and Catherine to benefit from the Allans' wealth. They assume if they aren't wealthy now, they will be.

On hearing Mr Morland agreed to the marriage, Isabella launches into estacies of social and material advancement with a carriage and jewellery.
I always enjoy the last scene in Ch. 16 when Isabella receives the second letter explaining the circumstances of the arrangement.
Isabella and Mrs Thorpe are confronted with James' real material expectations - really quite good when one considers Isabella has no dowry or connections- yet we see the shock and disappointment Isabella attempts to cover in front of Catherine,

'Ah ! my Catherine you have found me out. That's the sting. The long, long endless two years and half that are to pass before your brother can hold the living.' (ch. 16)

I think we see Isabella's desperation here, not just the material girl. She is aged 22, in competion with younger women with dowries, insecure and feels a need for speed to gain an establishment. Now Isabella has achived what she wanted, namely, an honourable engagement. Yet she doesn't seem so grateful for her luck, does she ?
I agree Isabella Thorpe deserves a SUTH ! I think she is typical of those shallow young women JA observed who compete in the social arena for the highest reward.
Grr !