I don't know about her motives but I think she really loves William. If it was about money, she could've done a Pippa and married a rich aristocrat
They were together for 7 years before the engagement. During that time, the press harassed her and her family, the paparazzis were relentless. Then, it didn't really get better after marriage. She got a lot of bad press. There's a great article online about the coverage of Kate before Meghan and after Meghan. It is only when Meghan arrived that the press layed off because they had a new target.
So, pretty much 15 years of horrible coverage with people accusing you of having an ED, the constant public pressure, being under the spotlight...you couldn't pay me to go through that.
Also, I think that the "Kate is desperate to be Queen" is nonsense.
She moved her family from a palace to a much smaller home in the countryside. She drops her kids off to school and picks them up everyday. She doesn't work during the school breaks.
To me, it always looked like her dream life was to be a stay-at-home mum living a quiet life in the countryside